Before validating a STUN packet, check it's big enough for a header.

Otherwise, we'll read out of bounds if the packet is too small.

NOTRY=true

Review-Url: https://codereview.webrtc.org/2040953003
Cr-Commit-Position: refs/heads/master@{#13093}

webrtc/p2p/base/stun.cc

@@ -132,7 +132,7 @@ const StunUInt16ListAttribute* StunMessage::GetUnknownAttributes() const {
 bool StunMessage::ValidateMessageIntegrity(const char* data, size_t size,
                                            const std::string& password) {
   // Verifying the size of the message.
-  if ((size % 4) != 0) {
+  if ((size % 4) != 0 || size < kStunHeaderSize) {
     return false;
   }