Now memcpy is avoided for NAL units that don't contain 0x03 escape
characters.

Improves performance of cbs_h2645_fragment_add_nals from 36940
decicycles to 6364 decicycles based on 8 runs with a 5.1 Mb/s H.264
sample (262144 runs each).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>