Assert(PqCommReadingMsg) in pq_peekbyte().

Interrupting pq_recvbuf() can break protocol sync, so its callers all
deserve this assertion.  The one pq_peekbyte() caller suffices already.

src/backend/libpq/pqcomm.c

@@ -928,6 +928,8 @@ pq_getbyte(void)
 int
 pq_peekbyte(void)
 {
+	Assert(PqCommReadingMsg);
+
 	while (PqRecvPointer >= PqRecvLength)
 	{
 		if (pq_recvbuf())		/* If nothing in buffer, then recv some */