avcodec/pgssubdec: Fix left shift of 255 by 24 places cannot be represented in type int

Fixes: b293a6479bb4b5286cff24d356bfd955/asan_generic_225c3c9_7819_cc526b657450c6cdef1371b526499626.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pgssubdec.c

@@ -33,7 +33,7 @@
 #include "libavutil/imgutils.h"
 #include "libavutil/opt.h"
 
-#define RGBA(r,g,b,a) (((a) << 24) | ((r) << 16) | ((g) << 8) | (b))
+#define RGBA(r,g,b,a) (((unsigned)(a) << 24) | ((r) << 16) | ((g) << 8) | (b))
 #define MAX_EPOCH_PALETTES 8   // Max 8 allowed per PGS epoch
 #define MAX_EPOCH_OBJECTS  64  // Max 64 allowed per PGS epoch
 #define MAX_OBJECT_REFS    2   // Max objects per display set