varlinkctl: when operating in --more mode, fail correcly on Varlink method error

In varlink.c we generally do not make failing callback functions fatal,
since that should be up to the app. Hence, in case of varlinkctl (where
we want failures to be fatal), make sure to propagate the error back
explicitly.

Before this change a failing call to "varlinkctl --more call …" would result in
a zero exit code. With this it will correctly exit with a non-zero exit
code.

src/varlinkctl/varlinkctl.c

@@ -350,7 +350,7 @@ static int reply_callback(
                 VarlinkReplyFlags flags,
                 void *userdata)  {
 
-        int r;
+        int *ret = ASSERT_PTR(userdata), r;
 
         assert(link);
 
@@ -358,7 +358,7 @@ static int reply_callback(
                 /* Propagate the error we received via sd_notify() */
                 (void) sd_notifyf(/* unset_environment= */ false, "VARLINKERROR=%s", error);
 
-                r = log_error_errno(SYNTHETIC_ERRNO(EBADE), "Method call failed: %s", error);
+                r = *ret = log_error_errno(SYNTHETIC_ERRNO(EBADE), "Method call failed: %s", error);
         } else
                 r = 0;
 
@@ -432,7 +432,8 @@ static int verb_call(int argc, char *argv[], void *userdata) {
 
         } else if (arg_method_flags & VARLINK_METHOD_MORE) {
 
-                varlink_set_userdata(vl, (void*) method);
+                int ret = 0;
+                varlink_set_userdata(vl, &ret);
 
                 r = varlink_bind_reply(vl, reply_callback);
                 if (r < 0)
@@ -459,6 +460,8 @@ static int verb_call(int argc, char *argv[], void *userdata) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to wait for varlink connection events: %m");
                 }
+
+                return ret;
         } else {
                 JsonVariant *reply = NULL;
                 const char *error = NULL;