avformat/h263dec: Fix h263 probe

The code was missing 1 bit in the src format
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/h263dec.c

@@ -35,7 +35,7 @@ static int h263_probe(AVProbeData *p)
     for(i=0; i<p->buf_size; i++){
         code = (code<<8) + p->buf[i];
         if ((code & 0xfffffc0000) == 0x800000) {
-            src_fmt= (code>>2)&3;
+            src_fmt= (code>>2)&7;
             if(   src_fmt != last_src_fmt
                && last_src_fmt>0 && last_src_fmt<6
                && src_fmt<6)