return a null pointer at the end of QuicSession::ProcessPendingStream; should be unreachable, but will appease some compilers.

PiperOrigin-RevId: 574588517

quiche/quic/core/quic_session.cc

diff --git a/quiche/quic/core/quic_session.cc b/quiche/quic/core/quic_session.cc
index f9811ff..4daff51 100644
--- a/quiche/quic/core/quic_session.cc
+++ b/quiche/quic/core/quic_session.cc
@@ -2775,6 +2775,8 @@
           << stream_type;
       return nullptr;
   }
+  return nullptr;  // Unreachable, unless the enum value is out-of-range
+                   // (potentially undefined behavior)
 }
 
 #undef ENDPOINT  // undef for jumbo builds