Fix invalid read in virNetSASLSessionClientStep debug log
    
    virNetSASLSessionClientStep logs the data that is going to be passed to
    sasl_client_step as input data. However, it tries to log it as a string,
    while there is no guarantee that this data is going to be nul-terminated.
    This leads to this valgrind log:
    
    ==20938== Invalid read of size 1
    ==20938==    at 0x8BDB08F: vfprintf (vfprintf.c:1635)
    ==20938==    by 0x8C06DF2: vasprintf (vasprintf.c:62)
    ==20938==    by 0x4CCEDF9: virVasprintfInternal (virstring.c:337)
    ==20938==    by 0x4CA9516: virLogVMessage (virlog.c:842)
    ==20938==    by 0x4CA939A: virLogMessage (virlog.c:778)
    ==20938==    by 0x4E21E0D: virNetSASLSessionClientStep (virnetsaslcontext.c:458)
    ==20938==    by 0x4DE47B8: remoteAuthSASL (remote_driver.c:4136)
    ==20938==    by 0x4DE33AE: remoteAuthenticate (remote_driver.c:3635)
    ==20938==    by 0x4DDBFAA: doRemoteOpen (remote_driver.c:832)
    ==20938==    by 0x4DDC8BA: remoteConnectOpen (remote_driver.c:1027)
    ==20938==    by 0x4D8595F: do_open (libvirt.c:1239)
    ==20938==    by 0x4D863F3: virConnectOpenAuth (libvirt.c:1481)
    ==20938==    by 0x12762B: vshReconnect (virsh.c:337)
    ==20938==    by 0x12C9B0: vshInit (virsh.c:2470)
    ==20938==    by 0x12E9A5: main (virsh.c:3338)
    ==20938==  Address 0xe329ccd is 0 bytes after a block of size 141 alloc'd
    ==20938==    at 0x4A081D4: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==20938==    by 0x8CB91B4: xdr_array (xdr_array.c:94)
    ==20938==    by 0x4E039C2: xdr_remote_auth_sasl_start_ret (remote_protocol.c:3134)
    ==20938==    by 0x4E1F8AA: virNetMessageDecodePayload (virnetmessage.c:405)
    ==20938==    by 0x4E119F5: virNetClientProgramCall (virnetclientprogram.c:377)
    ==20938==    by 0x4DF8141: callFull (remote_driver.c:5794)
    ==20938==    by 0x4DF821A: call (remote_driver.c:5816)
    ==20938==    by 0x4DE46CF: remoteAuthSASL (remote_driver.c:4112)
    ==20938==    by 0x4DE33AE: remoteAuthenticate (remote_driver.c:3635)
    ==20938==    by 0x4DDBFAA: doRemoteOpen (remote_driver.c:832)
    ==20938==    by 0x4DDC8BA: remoteConnectOpen (remote_driver.c:1027)
    ==20938==    by 0x4D8595F: do_open (libvirt.c:1239)
    ==20938==    by 0x4D863F3: virConnectOpenAuth (libvirt.c:1481)
    ==20938==    by 0x12762B: vshReconnect (virsh.c:337)
    ==20938==    by 0x12C9B0: vshInit (virsh.c:2470)
    ==20938==    by 0x12E9A5: main (virsh.c:3338)