avcodec/ffv1dec_template: Fix undefined shift

Fixes: runtime error: left shift of negative value -127
Fixes: 2834/clusterfuzz-testcase-minimized-5988039123795968

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffv1dec_template.c

@@ -149,7 +149,7 @@ static void RENAME(decode_rgb_frame)(FFV1Context *s, uint8_t *src[3], int w, int
             }
 
             if (lbd)
-                *((uint32_t*)(src[0] + x*4 + stride[0]*y)) = b + (g<<8) + (r<<16) + (a<<24);
+                *((uint32_t*)(src[0] + x*4 + stride[0]*y)) = b + ((unsigned)g<<8) + ((unsigned)r<<16) + ((unsigned)a<<24);
             else if (sizeof(TYPE) == 4) {
                 *((uint16_t*)(src[0] + x*2 + stride[0]*y)) = g;
                 *((uint16_t*)(src[1] + x*2 + stride[1]*y)) = b;