notify: add one more assert

We tend to get those preallocated array sizes wrong.
In this case, the count was correct, but add the usual
assert.

src/notify/notify.c

@@ -604,6 +604,7 @@ static int run(int argc, char* argv[]) {
         }
 
         our_env[i++] = NULL;
+        assert(i <= ELEMENTSOF(our_env));
 
         final_env = strv_env_merge((char**) our_env, arg_env);
         if (!final_env)