Use *array() allocation functions where appropriate

This prevents overflow from multiplication.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/usermod.c

@@ -2150,7 +2150,7 @@ int main (int argc, char **argv)
 #endif
 
 	sys_ngroups = sysconf (_SC_NGROUPS_MAX);
-	user_groups = (char **) malloc (sizeof (char *) * (1 + sys_ngroups));
+	user_groups = (char **) mallocarray (sys_ngroups + 1, sizeof (char *));
 	user_groups[0] = NULL;
 
 	is_shadow_pwd = spw_file_present ();