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

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

This is a complement to partial fix from commit
  debaa04f9e249f6bf75d40f38b34cfdcd7fc2047
  BUG/MINOR: qpack: abort on dynamic index field line decoding

The main objective is to fix coverity report about usage of
uninitialized variable when receiving dynamic table references. These
references are invalid as for the moment haproxy advertizes a 0-sized
dynamic table. An ABORT_NOW clause is present to catch this. A following
patch will clean up this in order to properly handle QPACK errors with
CONNECTION_CLOSE.

This should fix github issue #1753.

No need to backport as this was introduced in the current dev branch.