Guard HTTP/2 frame sequence lookahead (#364)

H2Session::write() pops the current frame from the stream frame
sequence and then immediately looks at the next entry to decide
which flags to set. When the current frame is the last queued
frame, that unconditional front() call reads past the end of the
deque and aborts proxy-verifier with a std::deque::front()
assertion.

Treat an empty post-pop frame sequence as having no next frame by
using H2Frame::INVALID for the lookahead value. That preserves the
existing flag selection logic while avoiding the invalid deque
access on the final frame.

Co-authored-by: bneradt <bneradt@yahooinc.com>

Author: bneradt

src/core/http2.cc

@@ -1505,7 +1505,9 @@ H2Session::write(HttpHeader const &hdr)
     while (!stream_state->_h2_frame_sequence.empty()) {
       auto curr_frame = stream_state->_h2_frame_sequence.front();
       stream_state->_h2_frame_sequence.pop_front();
-      auto next_frame = stream_state->_h2_frame_sequence.front();
+      auto const next_frame = stream_state->_h2_frame_sequence.empty() ?
+                                  H2Frame::INVALID :
+                                  stream_state->_h2_frame_sequence.front();
 
       uint8_t flags = 0;