swscale: fix sign extensions in yuv planar conversion

Casting the left-most byte to unsigned avoids an undefined
result of the shift by 24 if bit 7 is set.

yuvPlanartouyvy_c and yuvPlanartoyuy2_c are affected.

CC: libav-stable@libav.org
Bug-Id: CID 732281 / CID 732282

libswscale/rgb2rgb_template.c

@@ -376,9 +376,9 @@ static inline void yuvPlanartoyuy2_c(const uint8_t *ysrc, const uint8_t *usrc,
         const uint8_t *yc = ysrc, *uc = usrc, *vc = vsrc;
         for (i = 0; i < chromWidth; i += 2) {
             uint64_t k = yc[0] + (uc[0] << 8) +
-                         (yc[1] << 16) + (vc[0] << 24);
+                         (yc[1] << 16) + ((unsigned) vc[0] << 24);
             uint64_t l = yc[2] + (uc[1] << 8) +
-                         (yc[3] << 16) + (vc[1] << 24);
+                         (yc[3] << 16) + ((unsigned) vc[1] << 24);
             *ldst++ = k + (l << 32);
             yc     += 4;
             uc     += 2;
@@ -440,9 +440,9 @@ static inline void yuvPlanartouyvy_c(const uint8_t *ysrc, const uint8_t *usrc,
         const uint8_t *yc = ysrc, *uc = usrc, *vc = vsrc;
         for (i = 0; i < chromWidth; i += 2) {
             uint64_t k = uc[0] + (yc[0] << 8) +
-                         (vc[0] << 16) + (yc[1] << 24);
+                         (vc[0] << 16) + ((unsigned) yc[1] << 24);
             uint64_t l = uc[1] + (yc[2] << 8) +
-                         (vc[1] << 16) + (yc[3] << 24);
+                         (vc[1] << 16) + ((unsigned) yc[3] << 24);
             *ldst++ = k + (l << 32);
             yc     += 4;
             uc     += 2;