BUG/MAJOR: qpack: fix possible read out of bounds in static table

CertiK Skyfall Team reported that passing an index greater than
QPACK_SHT_SIZE in a qpack instruction referencing a literal field
name with name reference or and indexed field line will cause a
read out of bounds that may crash the process, and confirmed that
this fix addresses the issue.

This needs to be backported as far as 2.5.
1 file changed