src/usermod.c: update_gshadow_file(): Fix RESOURCE_LEAK (CWE-772)

src/usermod.c: update_gshadow_file(): Fix RESOURCE_LEAK (CWE-772)

Report:
> shadow-4.15.0/src/usermod.c:864:3: alloc_fn: Storage is returned from allocation function "__sgr_dup".
> shadow-4.15.0/src/usermod.c:864:3: var_assign: Assigning: "nsgrp" = storage returned from "__sgr_dup(sgrp)".
> shadow-4.15.0/src/usermod.c:964:1: leaked_storage: Variable "nsgrp" going out of scope leaks the storage it points to.
> 962|                   free (nsgrp);
> 963|           }
> 964|-> }
> 965|   #endif                                /* SHADOWGRP */
> 966|

Link: https://issues.redhat.com/browse/RHEL-35383
Reported-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>