ldap_child: Fix use after free
    
    In case on any krb5 related error, we tried to send string
    interpretation of krb5 error tb parrent in prepare_response.
    
    However, we cannot use global krb5 context (krb5_error_ctx)
    because the context is every time released in done section of
    ldap_child_get_tgt_sync.
    
    This patch rather return duplicated string to prevent use after free.
    
    Backtrace:
     #0  __strchr_sse42 () at ../sysdeps/x86_64/multiarch/strchr.S:100
     100     ../sysdeps/x86_64/multiarch/strchr.S: No such file or directory.
    
     Thread 1 (Thread 0x7fc96cad5880 (LWP 11201)):
     #0  __strchr_sse42 () at ../sysdeps/x86_64/multiarch/strchr.S:100
     No locals.
     #1  0x00007fc96be43725 in err_fmt_fmt (msg=0x7fc96d1cf8d0 "Cannot find KDC for requested realm",
                                            code=-1765328230,
                                            err_fmt=<optimized out>) at kerrs.c:152
            buf = {buftype = K5BUF_DYNAMIC, data = 0x7fc96d1cdb10,
                   space = 128, len = 0}
            p = <optimized out>
            s = 0xdededededededede <Address 0xdededededededede out of bounds>
     #2  krb5_get_error_message (ctx=<optimized out>,
                                 code=code@entry=-1765328230) at kerrs.c:184
            std = 0x7fc96d1cf8d0 "Cannot find KDC for requested realm"
     #3  0x00007fc96cb224e5 in sss_krb5_get_error_message (ctx=<optimized out>,
                                                           ec=ec@entry=-1765328230) at src/util/sss_krb5.c:424
     No locals.
     #4  0x00007fc96cb1fbb0 in prepare_response (rsp=<synthetic pointer>,
                                                 kerr=-1765328230, expire_time=0,
                                                 ccname=0x0,
                                                 mem_ctx=0x7fc96d1cb390) at src/providers/ldap/ldap_child.c:553
            ret = <optimized out>
            r = 0x7fc96d1cd8b0
            krb5_msg = 0x0
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>