BUG/MINOR: qpack: abort on dynamic index field line decoding

BUG/MINOR: qpack: abort on dynamic index field line decoding

Add an ABORT_NOW() clause if indexed field line referred to the dynamic
table. This is required as current haproxy QPACK implementation does not
support the dynamic table.

Note that this should not happen as haproxy explicitely advertizes a
null-sized dynamic table to the other peer.

This shoud fix github issue #1753.

No need to backport as this was introduced by commit
  b666c6b26eae3f17eee058eb6bcc9fe1b1c304d2
  MINOR: qpack: improve decoding function