lavc: avoid invalid memcpy() in avcodec_default_release_buffer()

When the buf and last pointers are equal, the FFSWAP() results
in an invalid call to memcpy() with same source and destination
on some targets.  Although assigning a struct to itself is valid
C99, gcc does not check for this before calling memcpy().
See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667

Signed-off-by: Mans Rullgard <mans@mansr.com>
