issue #1734, avoid passing null ptr to memmove

src/iobuf.c

@@ -28,7 +28,7 @@ int mg_iobuf_resize(struct mg_iobuf *io, size_t 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) memmove(p, io->buf, len);
+      if (len > 0 && io->buf != NULL) memmove(p, io->buf, len);
       zeromem(io->buf, io->size);
       free(io->buf);
       io->buf = (unsigned char *) p;