buffer: fix av_buffer_realloc() when the data is offset wrt buffer start

buffer: fix av_buffer_realloc() when the data is offset wrt buffer start

In such a case behave as if the buffer was not reallocatable -- allocate a
new one and copy the data (preserving just the part described by the
reference passed to av_buffer_realloc).

CC: libav-stable@libav.org
Reported-By: wm4 <nfxjfg@googlemail.com>