multiplexer: fix consume of too many bytes (#11499)

IOBufferReader::consume contractually assumes that all callers of it
will not consume more bytes than read_avail for the buffer chain. This
adds a debug assertion for this and fixes multiplexer so that it doesn't
violate this invariant.

plugins/multiplexer/fetcher.h

@@ -213,6 +213,8 @@ template <class T> struct HttpTransaction {
             self->t_.header(self->parser_.buffer_, self->parser_.location_);
             self->parsingHeaders_ = false;
           }
+          // Parsing headers will indirectly read from our reader. Update available accordingly.
+          available = TSIOBufferReaderAvail(self->in_->reader);
         }
         if (!self->parsingHeaders_) {
           if (self->chunkDecoder_ != NULL) {

src/iocore/eventsystem/P_IOBuffer.h

@@ -551,6 +551,7 @@ IOBufferReader::is_read_avail_more_than(int64_t size)
 TS_INLINE void
 IOBufferReader::consume(int64_t n)
 {
+  ink_assert(read_avail() >= n);
   start_offset += n;
   if (size_limit != INT64_MAX) {
     size_limit -= n;