useradd: free grp to avoid leak

useradd: free grp to avoid leak

covscan issue:
Error: RESOURCE_LEAK (CWE-772): [#def39] [important]
src/useradd.c:728: alloc_fn: Storage is returned from allocation function "get_local_group".
src/useradd.c:728: var_assign: Assigning: "grp" = storage returned from "get_local_group(list)".
src/useradd.c:728: overwrite_var: Overwriting "grp" in "grp = get_local_group(list)" leaks the storage that "grp" points to.
726|   		 * GID values, otherwise the string is looked up as is.
727|   		 */
728|-> 		grp = get_local_group (list);
729|
730|   		/*