Description of problem:
sssd crashes during start if id_provider is not mentioned

Version-Release number of selected component (if applicable):
sssd-1.9.2-30.el6

How reproducible:
Always

Steps to Reproduce:
1. Remove the line "id_provider=..." from domain section of sssd.conf

2. Start sssd
# service sssd start
Starting sssd: /bin/bash: line 1:  3096 Segmentation fault      (core dumped)
/usr/sbin/sssd -f -D
                                                           [FAILED]


Actual results:
sssd crashes

Expected results:
sssd should not crash. Instead should fail to start and appropriate error
should be logged in sssd.log

Additional info:
Backtrace:
# gdb --core=coredump /usr/sbin/sssd --quiet -ex "thread apply all bt full" -ex
"quit"
Core was generated by `/usr/sbin/sssd -f -D'.
Program terminated with signal 11, Segmentation fault.
#0  __strcasecmp_l_sse2 () at ../sysdeps/x86_64/strcmp.S:213
213             movlpd  (%rdi), %xmm1

Thread 1 (Thread 0x7f8a3a858700 (LWP 2878)):
#0  __strcasecmp_l_sse2 () at ../sysdeps/x86_64/strcmp.S:213
No locals.
#1  0x000000000041052f in add_implicit_services (ctx=0x1e59470)
    at src/monitor/monitor.c:858
        ret = <value optimized out>
        tmp_ctx = 0x1e5eb70
        c = <value optimized out>
        id_provider = 0x0
        domain_names = 0x1e72cf0
        conf_path = <value optimized out>
        add_pac = <value optimized out>
#2  get_monitor_config (ctx=0x1e59470) at src/monitor/monitor.c:938
        ret = <value optimized out>
        timeout_seconds = 10
        badsrv = 0x0
        i = <value optimized out>
        __FUNCTION__ = "get_monitor_config"
Missing separate debuginfos, use: debuginfo-install
cyrus-sasl-lib-2.1.23-13.el6.x86_64 nss-softokn-freebl-3.12.9-11.el6.x86_64
zlib-1.2.3-29.el6.x86_64
---Type <return> to continue, or q <return> to quit---
#3  0x00000000004131c3 in load_configuration (argc=31822968,
    argv=<value optimized out>) at src/monitor/monitor.c:1610
        ret = 0
        ctx = 0x1e59470
        cdb_file = 0x0
#4  main (argc=31822968, argv=<value optimized out>)
    at src/monitor/monitor.c:2755
        opt = <value optimized out>
        pc = <value optimized out>
        opt_daemon = 1
        opt_interactive = 0
        opt_version = 0
        opt_config_file = 0x0
        config_file = 0x1e59390 "/etc/sssd/sssd.conf"
        flags = 5
        main_ctx = <value optimized out>
        tmp_ctx = 0x1e59330
        monitor = <value optimized out>
        ret = <value optimized out>
        uid = <value optimized out>
        long_options = {{longName = 0x0, shortName = 0 '\000', argInfo = 4,
            arg = 0x669840, val = 0, descrip = 0x458863 "Help options:",
            argDescrip = 0x0}, {longName = 0x458871 "debug-level",
---Type <return> to continue, or q <return> to quit---
            shortName = 100 'd', argInfo = 2, arg = 0x669920, val = 0,
            descrip = 0x45880a "Debug level", argDescrip = 0x0}, {
            longName = 0x457c54 "debug-to-files", shortName = 102 'f',
            argInfo = 0, arg = 0x669924, val = 0,
            descrip = 0x459878 "Send the debug output to files instead of
stderr", argDescrip = 0x0}, {longName = 0x45887d "debug-timestamps",
            shortName = 0 '\000', argInfo = 2, arg = 0x669698, val = 0,
            descrip = 0x458816 "Add debug timestamps", argDescrip = 0x0}, {
            longName = 0x45888e "debug-microseconds", shortName = 0 '\000',
            argInfo = 2, arg = 0x66969c, val = 0,
            descrip = 0x4598b0 "Show timestamps with microseconds",
            argDescrip = 0x0}, {longName = 0x4588a1 "daemon",
            shortName = 68 'D', argInfo = 0, arg = 0x7fff2f5bc34c, val = 0,
            descrip = 0x45882b "Become a daemon (default)", argDescrip = 0x0},
          {longName = 0x4588a8 "interactive", shortName = 105 'i',
            argInfo = 0, arg = 0x7fff2f5bc348, val = 0,
            descrip = 0x4598d8 "Run interactive (not a daemon)",
            argDescrip = 0x0}, {longName = 0x45b2bf "config",
            shortName = 99 'c', argInfo = 1, arg = 0x7fff2f5bc338, val = 0,
            descrip = 0x4598f8 "Specify a non-default config file",
            argDescrip = 0x0}, {longName = 0x45af2b "version",
            shortName = 0 '\000', argInfo = 0, arg = 0x7fff2f5bc344, val = 0,
            descrip = 0x458845 "Print version number and exit",
---Type <return> to continue, or q <return> to quit---
            argDescrip = 0x0}, {longName = 0x0, shortName = 0 '\000',
            argInfo = 0, arg = 0x0, val = 0, descrip = 0x0, argDescrip = 0x0}}
        __FUNCTION__ = "main"