mxqkill: Add missing initialization

If the -u option is not given, passwd will not be set in the args
evaluation loop, yet we use `if (!passwd) {` later in the code.

Initialze pointer.

mxqkill.c

@@ -238,7 +238,7 @@ int main(int argc, char *argv[])
     struct mxq_group group;
 
     uid_t ruid, euid, suid;
-    struct passwd *passwd;
+    struct passwd *passwd = NULL;
 
     int res;