swscale/output: Fix integer overflow in alpha computation in yuv2gbrp16_full_X_c()

Fixes: signed integer overflow: 524280 * 4432 cannot be represented in type 'int'
Fixes: ticket8322

Found-by: Suhwan
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 49ba1879ad)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/output.c

@@ -2062,7 +2062,7 @@ yuv2gbrp16_full_X_c(SwsContext *c, const int16_t *lumFilter,
             A = -0x40000000;
 
             for (j = 0; j < lumFilterSize; j++)
-                A += alpSrc[j][i] * lumFilter[j];
+                A += alpSrc[j][i] * (unsigned)lumFilter[j];
 
             A >>= 1;
             A += 0x20002000;