[Cast channel fuzzer] Add size check in the fuzzer code.

-max_len=N doesn't guarantee that libfuzzer won't ever try feeding a
larger input into the target. So a check is added to the code to enforce
the size limit.

Bug: 828359
Change-Id: I2aa1404e618350896636de42a0ecc426946c8401
Reviewed-on: https://chromium-review.googlesource.com/996403
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548174}

components/cast_channel/cast_message_fuzzer.cc

@@ -20,6 +20,9 @@ google::protobuf::LogSilencer log_silencer;
 namespace cast_channel {
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  if (size > MessageFramer::MessageHeader::max_message_size())
+    return 0;
+
   scoped_refptr<net::GrowableIOBuffer> buffer =
       base::MakeRefCounted<net::GrowableIOBuffer>();
   buffer->SetCapacity(MessageFramer::MessageHeader::max_message_size());