Handle exec error correctly

exec(1) states that exec will only return in case of an error,
and the return value is -1. Therefore, if (!execv(...)) doesn't
make sense.

xdg-app-builtins-run.c

@@ -289,7 +289,7 @@ xdg_app_builtin_run (int argc, char **argv, GCancellable *cancellable, GError **
   g_unsetenv ("LD_LIBRARY_PATH");
   g_setenv ("PATH", "/self/bin:/usr/bin", TRUE);
 
-  if (!execv (HELPER, (char **)argv_array->pdata))
+  if (execv (HELPER, (char **)argv_array->pdata) == -1)
     {
       g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno), "Unable to start app");
       goto out;

xdg-app-helper.c

@@ -1156,5 +1156,8 @@ main (int argc,
 
   __debug__(("launch executable %s\n", args[0]));
 
-  return execvp (args[0], args);
+  if (execvp (args[0], args) == -1)
+    die_with_error ("execvp %s", args[0]);
+
+  return 0;
 }