oggparsevorbis: use av_realloc consistently

Memory passed to av_realloc cannot be allocated using memalign.

From realloc(3):

The realloc() function changes the size of the memory block pointed to
by ptr to size bytes. (...) Unless ptr is NULL, it must have been returned
by an earlier call to malloc(), calloc() or realloc().

The issue has been found by debugallocation, a part of google-perftools:
http://code.google.com/p/gperftools/ .

Signed-off-by: Paweł Hajdan, Jr <phajdan@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
fixup_vorbis_headers: add missing malloc failure check

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