dump_extradata: Insert extradata even for small packets

dump_extradata: Insert extradata even for small packets

3469cfab added a check for whether the extradata coincided with the
beginning of the packet's data in order not to add extradata to packets
that already have it. But the check used was buggy for packets whose
size is smaller than the extradata's size. This commit fixes this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>