fix iobuf shrink to less than current len

mongoose.c

@@ -2608,6 +2608,7 @@ int mg_iobuf_resize(struct mg_iobuf *io, size_t new_size) {
       mg_free(io->buf);
       io->buf = (unsigned char *) p;
       io->size = new_size;
+      io->len = len;
     } else {
       ok = 0;
       MG_ERROR(("%lld->%lld", (uint64_t) io->size, (uint64_t) new_size));

src/iobuf.c

@@ -25,6 +25,7 @@ int mg_iobuf_resize(struct mg_iobuf *io, size_t new_size) {
       mg_free(io->buf);
       io->buf = (unsigned char *) p;
       io->size = new_size;
+      io->len = len;
     } else {
       ok = 0;
       MG_ERROR(("%lld->%lld", (uint64_t) io->size, (uint64_t) new_size));

test/unit_test.c

@@ -322,6 +322,8 @@ static void test_iobuf(void) {
   mg_iobuf_add(&io, io.len, "a", 1);
   ASSERT(io.buf != NULL && io.size == 20 && io.len == 11);
   ASSERT(memcmp(io.buf, "xhi!123456a", io.len) == 0);
+  mg_iobuf_resize(&io, 1);
+  ASSERT(io.buf != NULL && io.size == 10 && io.len == 10);
   mg_iobuf_free(&io);
 }