core: check if argc > 0 and argv[0] is set

Follow-up for 1637e75707.

src/core/main.c

@@ -2726,6 +2726,8 @@ int main(int argc, char *argv[]) {
         Manager *m = NULL;
         FDSet *fds = NULL;
 
+        assert(argc > 0 && !isempty(argv[0]));
+
         /* SysV compatibility: redirect init → telinit */
         redirect_telinit(argc, argv);