BUG/MINOR: qpack: accept a Required Insert Count which cannot be reached

qpack_decode_fs() parses the field section prefix but never looks at the
result. haproxy advertises a null dynamic table capacity, so a conformant
encoder cannot produce a non null Encoded Required Insert Count, and RFC 9204
4.5.1.1 asks for QPACK_DECOMPRESSION_FAILED in that case. It is now rejected.

Nothing could be done with the accepted prefix, as every field line
representation referencing the dynamic table is already rejected, so this is a
compliance fix rather than a security one.

This should be backported as far as 2.6.