[gnutls-devel] GnuTLS | mbuffer_st.type is not preserved when linearized (#1319)

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Tatsuhiro Tsujikawa created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1319



## Description of problem:
When received data is too short to contain a message header, they are temporarily buffered in somewhere and when the next segment is arrived, they are gathered into one buffer (linearized).  I'm specifically looking at this code:
https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1259

The problem is, when the buffers are linearized, mbuffer_st type is cleared.  Next time we hit https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1217, the check fails, and the endpoint sends fatal alert with unexpected message.

## Version of gnutls used:
3.7.3

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)


## How reproducible:

Steps to Reproduce:

I think feeding data ends in the middle of a message header to gnutls_session_t via gnutls_handshake_write would trigger this bug.
For myself, I discovered it when testing client certificate authentication as QUIC client.

## Actual results:


## Expected results:

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