stm32f7 example starts on real h/w, with plus-TCP driver

src/iobuf.c

@@ -19,9 +19,9 @@ int mg_iobuf_resize(struct mg_iobuf *io, size_t new_size) {
     io->buf = NULL;
     io->len = io->size = 0;
   } else if (new_size != io->size) {
-    // NOTE(lsm): do not use realloc here. Use malloc/free only, to ease the
+    // NOTE(lsm): do not use realloc here. Use calloc/free only, to ease the
     // porting to some obscure platforms like FreeRTOS
-    void *p = malloc(new_size);
+    void *p = calloc(1, new_size);
     if (p != NULL) {
       size_t len = new_size < io->len ? new_size : io->len;
       if (len > 0) memcpy(p, io->buf, len);