lavc: call ff_init_buffer_info() from default reget buffer.

This is needed in case the get_buffer() callback doesnt set
width/height.
Ideally all decoders would make calls through some wraper
to the callbacks and that wraper would call ff_init_buffer_info()
But until thats done, the default reget buffer must call this
itself as it needs the values for the changed size check later.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
