nss: remove unused label
    
    After 4937f2c6, Sumit noticed the following warning/breakage:
    
        make[2]: Leaving directory '/home/sbose/sssd/master_build/src/man'
        Making check in .
        make[2]: Entering directory '/home/sbose/sssd/master_build'
          CC       src/responder/nss/nss_protocol_grent.o
        ../src/responder/nss/nss_protocol_grent.c: In function 'nss_protocol_fill_initgr':
        ../src/responder/nss/nss_protocol_grent.c:409:1: error: label 'done' defined but not used [-Werror=unused-label]
         done:
         ^~~~
        cc1: all warnings being treated as errors
        Makefile:17808: recipe for target 'src/responder/nss/nss_protocol_grent.o' failed
        make[2]: *** [src/responder/nss/nss_protocol_grent.o] Error 1
        make[2]: Leaving directory '/home/sbose/sssd/master_build'
    
    Also, while removing the label, by moving the error treatment to the if
    block just before the existing one makes the code cleaner.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>