fix mg_send() failing on 0 length

test/mip_tap_test.c

@@ -413,6 +413,7 @@ static void test_http_server(struct mg_mgr *mgr) {
       mg_mprintf("./mip_curl.sh http://%M:12347", mg_print_ip4, &mgr->ifp->ip);
 #endif
   ASSERT(c != NULL);
+  ASSERT (mg_send(c, "NADA", 0)); // check mg_send allows len=0
   pthread_create(&thread_id, NULL, poll_thread,
                  mgr);  // simpler this way, no concurrency anyway
   MG_DEBUG(("CURL"));

test/unit_test.c

@@ -2977,6 +2977,14 @@ static void eZ(struct mg_connection *c, int ev, void *ev_data) {
   (void) ev_data;
 }
 
+static void eS(struct mg_connection *c, int ev, void *ev_data) {
+  if (ev == MG_EV_HTTP_MSG) {
+    ASSERT (mg_send(c, "NADA", 0));
+    mg_http_reply(c, 200, "", "abcd");
+  }
+  (void) ev_data;
+}
+
 // Do not delete chunks as they arrive
 static void eh4(struct mg_connection *c, int ev, void *ev_data) {
   uint32_t *crc = (uint32_t *) c->fn_data;
@@ -3012,6 +3020,9 @@ static void test_http_chunked_case(mg_event_handler_t s, mg_event_handler_t c,
 }
 
 static void test_http_chunked(void) {
+  // test mg_send allows calls with 0 length
+  test_http_chunked_case(eS, eh4, 1, "abcd");
+
   // Non-chunked encoding
   test_http_chunked_case(eY, eh4, 1, "abcd");  // Chunks not deleted
   test_http_chunked_case(eY, eh4, 2, "abcdabcd");