h264parse: May break stream further rather then fixing it

Submitted by Nicolas Dufresne `@ndufresne`

As found with this file in bug 787795, with the following FLV file:

h264parse turns a slightly broken stream into an un-playable stream. As an example, the display video will be gray with the parser:

filesrc location=test.flv ! flvdemux ! h264parse ! avdec_h264 ! autovideosink

But looks fine without:

filesrc location=/tmp/test.flv ! flvdemux ! video/x-h264,alignment=au ! avdec_h264 ! autovideosink