BUG/MINOR: h3: properly handle incomplete remote uni stream type

BUG/MINOR: h3: properly handle incomplete remote uni stream type

A H3 unidirectional stream is always opened with its stream type first
encoded as a QUIC variable integer. If the STREAM frame contains some
data but not enough to decode this varint, haproxy would crash due to an
ABORT_NOW() statement.

To fix this, ensure we support an incomplete stream type. In this case,
h3_init_uni_stream() returns 0 and the buffer content is not cleared.
Stream decoding will resume when new data are received for this stream
which should be enough to decode the stream type varint.

This bug has never occured on production because standard H3 stream types
are small enough to be encoded on a single byte.

This should be backported up to 2.6.