avcodec/dvdsubdec: Fixes 2 runtime error: left shift of 170 by 24 places cannot be represented in type 'int'

Fixes: 619/clusterfuzz-testcase-5803914534322176

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

libavcodec/dvdsubdec.c

@@ -189,12 +189,12 @@ static void guess_palette(DVDSubContext* ctx,
                 r = (((subtitle_color >> 16) & 0xff) * level) >> 8;
                 g = (((subtitle_color >> 8) & 0xff) * level) >> 8;
                 b = (((subtitle_color >> 0) & 0xff) * level) >> 8;
-                rgba_palette[i] = b | (g << 8) | (r << 16) | ((alpha[i] * 17) << 24);
+                rgba_palette[i] = b | (g << 8) | (r << 16) | ((alpha[i] * 17U) << 24);
                 color_used[colormap[i]] = (i + 1);
                 j++;
             } else {
                 rgba_palette[i] = (rgba_palette[color_used[colormap[i]] - 1] & 0x00ffffff) |
-                                    ((alpha[i] * 17) << 24);
+                                    ((alpha[i] * 17U) << 24);
             }
         }
     }