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>

libavutil/buffer.c

@@ -170,7 +170,7 @@ int av_buffer_realloc(AVBufferRef **pbuf, int size)
         return 0;
 
     if (!(buf->buffer->flags & BUFFER_FLAG_REALLOCATABLE) ||
-        !av_buffer_is_writable(buf)) {
+        !av_buffer_is_writable(buf) || buf->data != buf->buffer->data) {
         /* cannot realloc, allocate a new reallocable buffer and copy data */
         AVBufferRef *new = NULL;