codecparsers_h26x: Should return NO_NAL_END on 5/6 bytes NALs

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#797371)

Description

-- regression introduce by https://bugzilla.gnome.org/show_bug.cgi?id=732553 ---

If you pass any valid 5 first bytes of a NAL to the parser, it will now always return OK instead of NO_NAL_END. This is caused by broken code trying to handle single byte SEQ_END / STREAM_END as being complete NAL.