avcodec/svq1dec: Fix undefined shifts

Found-by: Clang -fsanitize=shift
Reported-by: Thierry Foucu <tfoucu@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/svq1dec.c

@@ -162,7 +162,8 @@ static int svq1_decode_block_intra(GetBitContext *bitbuf, uint8_t *pixels,
     const uint32_t *codebook;
     int entries[6];
     int i, j, m, n;
-    int mean, stages;
+    int stages;
+    unsigned mean;
     unsigned x, y, width, height, level;
     uint32_t n1, n2, n3, n4;
 
@@ -228,7 +229,8 @@ static int svq1_decode_block_non_intra(GetBitContext *bitbuf, uint8_t *pixels,
     const uint32_t *codebook;
     int entries[6];
     int i, j, m, n;
-    int mean, stages;
+    int stages;
+    unsigned mean;
     int x, y, width, height, level;
     uint32_t n1, n2, n3, n4;