src/usermod,groups: use checked malloc

usermod.c:2165:24: warning: dereference of possibly-NULL ‘user_groups’ [CWE-690] [-Wanalyzer-possible-null-dereference]

src/usermod.c

@@ -2161,7 +2161,7 @@ int main (int argc, char **argv)
 #endif
 
 	sys_ngroups = sysconf (_SC_NGROUPS_MAX);
-	user_groups = MALLOC(sys_ngroups + 1, char *);
+	user_groups = XMALLOC(sys_ngroups + 1, char *);
 	user_groups[0] = NULL;
 
 	is_shadow_pwd = spw_file_present ();