ljpeg: Add predictor=0 support.

Fixed first half of ticket889
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpeg.h

@@ -127,6 +127,7 @@ static inline void put_marker(PutBitContext *p, int code)
 
 #define PREDICT(ret, topleft, top, left, predictor)\
     switch(predictor){\
+        case 0: ret= 0; break;\
         case 1: ret= left; break;\
         case 2: ret= top; break;\
         case 3: ret= topleft; break;\