avcodec/scpr: mask bits to prevent out of array read

Fixes: 1615/clusterfuzz-testcase-minimized-6625214647500800

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/scpr.c

@@ -488,7 +488,7 @@ static int decompress_i(AVCodecContext *avctx, uint32_t *dst, int linesize)
 
         if (avctx->bits_per_coded_sample == 16) {
             cx1 = (clr & 0x3F00) >> 2;
-            cx = (clr & 0xFFFFFF) >> 16;
+            cx = (clr & 0x3FFFFF) >> 16;
         } else {
             cx1 = (clr & 0xFC00) >> 4;
             cx = (clr & 0xFFFFFF) >> 18;
@@ -726,7 +726,7 @@ static int decompress_p(AVCodecContext *avctx,
 
                     if (avctx->bits_per_coded_sample == 16) {
                         cx1 = (clr & 0x3F00) >> 2;
-                        cx = (clr & 0xFFFFFF) >> 16;
+                        cx = (clr & 0x3FFFFF) >> 16;
                     } else {
                         cx1 = (clr & 0xFC00) >> 4;
                         cx = (clr & 0xFFFFFF) >> 18;