Harden size check during textblob deserialization

Check the text size read from a buffer should not exceed the size of
the input buffer. This is to avoid memory allocation errors such as
out of memory.

BUG=chromium:809200

Change-Id: I47824f6e8122bd550ee97ac83e2251b7725865e7
Reviewed-on: https://skia-review.googlesource.com/113289
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>

src/core/SkTextBlob.cpp

@@ -809,7 +809,7 @@ sk_sp<SkTextBlob> SkTextBlob::MakeFromBuffer(SkReadBuffer& reader) {
             return nullptr;
         }
         int textSize = pe.extended ? reader.read32() : 0;
-        if (textSize < 0) {
+        if (textSize < 0 || static_cast<size_t>(textSize) > reader.size()) {
             return nullptr;
         }