Remove pointless overflow check in pointer math

Fixes https://github.com/behdad/harfbuzz/issues/227

src/hb-blob.cc

@@ -104,7 +104,6 @@ hb_blob_create (const char        *data,
 
   if (!length ||
       length >= 1u << 31 ||
-      data + length < data /* overflows */ ||
       !(blob = hb_object_create<hb_blob_t> ())) {
     if (destroy)
       destroy (user_data);