avio: exit early in fill_buffer without read_packet

Fixes an invalid free() with ass in avi. The sample in bug 98 passes
parts of AVPacket.data as buffer for the AVIOContext. Since the packet
is quite large fill_buffer tries to reallocate the buffer before doing
nothing. Fixes bug 98.

libavformat/aviobuf.c

@@ -565,6 +565,10 @@ static void fill_buffer(AVIOContext *s)
     int len= s->buffer_size - (dst - s->buffer);
     int max_buffer_size = s->max_packet_size ? s->max_packet_size : IO_BUFFER_SIZE;
 
+    /* can't fill the buffer without read_packet, just set EOF if appropiate */
+    if (!s->read_packet && s->buf_ptr >= s->buf_end)
+        s->eof_reached = 1;
+
     /* no need to do anything if EOF already reached */
     if (s->eof_reached)
         return;