swscale/swscale: fix integer overflow

Should fix fate failure with clang ftrapv
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit c9c04512)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale.c

@@ -208,8 +208,9 @@ static void lumRangeToJpeg16_c(int16_t *_dst, int width)
 {
     int i;
     int32_t *dst = (int32_t *) _dst;
-    for (i = 0; i < width; i++)
-        dst[i] = (FFMIN(dst[i], 30189 << 4) * 4769 - (39057361 << 2)) >> 12;
+    for (i = 0; i < width; i++) {
+        dst[i] = ((int)(FFMIN(dst[i], 30189 << 4) * 4769U - (39057361 << 2))) >> 12;
+    }
 }
 
 static void lumRangeFromJpeg16_c(int16_t *_dst, int width)