rgb24toyv12: break out in the middle before out of array reads.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/rgb2rgb_template.c

@@ -2264,6 +2264,9 @@ static inline void RENAME(rgb24toyv12)(const uint8_t *src, uint8_t *ydst, uint8_
         ydst += lumStride;
         src  += srcStride;
 
+        if(y+1 == height)
+            break;
+
         for (i=0; i<chromWidth; i++) {
             unsigned int b = src[6*i+0];
             unsigned int g = src[6*i+1];