avcodec/s3tc: fix alpha decoding when dimensions are not a multiple of 4

Fix alpha position error for edge blocks of odd-dimensioned frames
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/s3tc.c

@@ -71,8 +71,10 @@ static inline void dxt1_decode_pixels(GetByteContext *gb, uint32_t *d,
             pixels >>= 2;
             alpha  >>= 4;
         }
-        for (; x<4; x++)
+        for (; x<4; x++) {
             pixels >>= 2;
+            alpha  >>= 4;
+        }
         d += qstride;
     }
 }