Fix fuzzing handling of too-large inputs. (#14353)

If we can't put it in a PacketBuffer, we can't do anything with it.

Fixes #14348

examples/all-clusters-app/linux/fuzzing-main.cpp

@@ -61,6 +61,11 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t * aData, size_t aSize)
  Transport::PeerAddress peerAddr;
  System::PacketBufferHandle buf =
  System::PacketBufferHandle::NewWithData(aData, aSize, /* aAdditionalSize = */ 0, /* aReservedSize = */ 0);
+ if (buf.IsNull())
+ {
+ // Too big; we couldn't represent this as a packetbuffer to start with.
+ return 0;
+ }
 
  // Ignoring the return value from OnMessageReceived, because we might be
  // passing it all sorts of garbage that will cause it to fail.