avio: detect implicit truncation by assert in avio_w*

This check is somewhat more lenient as would be ideal because we dont
know if the input is signed or unsigned
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/aviobuf.c

@@ -144,6 +144,7 @@ static void flush_buffer(AVIOContext *s)
 
 void avio_w8(AVIOContext *s, int b)
 {
+    av_assert2(b>=-128 && b<=255);
     *s->buf_ptr++ = b;
     if (s->buf_ptr >= s->buf_end)
         flush_buffer(s);