swscale/output: Fix undefined shifts

Fixes: da14e86d8462be6493eab16bc2d40f88/asan_heap-oob_204cfd2_528_cov_340150052_COMPRESS.BMP

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/output.c

@@ -1709,8 +1709,8 @@ yuv2rgb_full_1_c_template(SwsContext *c, const int16_t *buf0,
         int A = 0; //init to silence warning
         for (i = 0; i < dstW; i++) {
             int Y = buf0[i] << 2;
-            int U = (ubuf0[i] - (128<<7)) << 2;
-            int V = (vbuf0[i] - (128<<7)) << 2;
+            int U = (ubuf0[i] - (128<<7)) * 4;
+            int V = (vbuf0[i] - (128<<7)) * 4;
 
             if (hasAlpha) {
                 A = (abuf0[i] + 64) >> 7;