h264parser: Fix parsing of nal prefix

Nal prefix contains minimally 1 bit, and are empty if that bit
(svc_extension_flag) isn't set. The problem is that GStreamer was
expecting 4 bytes. Split the size check in two, and fix the header_bytes
calculation.
23 jobs for !2735 with h264parse-fix-nal-prefix in 1 minute and 8 seconds (queued for 2 seconds)
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