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

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

Report:
> shadow-4.15.0/src/usermod.c:734:3: alloc_fn: Storage is returned from allocation function "__gr_dup".
> shadow-4.15.0/src/usermod.c:734:3: var_assign: Assigning: "ngrp" = storage returned from "__gr_dup(grp)".
> shadow-4.15.0/src/usermod.c:815:1: leaked_storage: Variable "ngrp" going out of scope leaks the storage it points to.
> 813|                   gr_free(ngrp);
> 814|           }
> 815|-> }
> 816|
> 817|   #ifdef SHADOWGRP

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