avcodec/texturedsp: Fix multiple runtime error: left shift of 255 by 24 places cannot be represented in type 'int'

Fixes: 1386/clusterfuzz-testcase-minimized-5323086394032128

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

libavcodec/texturedsp.c

@@ -35,7 +35,7 @@
 #define RGBA(r, g, b, a) (((uint8_t)(r) <<  0) | \
                           ((uint8_t)(g) <<  8) | \
                           ((uint8_t)(b) << 16) | \
-                          ((uint8_t)(a) << 24))
+                          ((unsigned)(uint8_t)(a) << 24))
 
 static av_always_inline void extract_color(uint32_t colors[4],
                                            uint16_t color0,