avcodec/utils: Fix undefined behavior in avpriv_toupper4

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -3454,7 +3454,7 @@ unsigned int avpriv_toupper4(unsigned int x)
     return av_toupper(x & 0xFF) +
           (av_toupper((x >>  8) & 0xFF) << 8)  +
           (av_toupper((x >> 16) & 0xFF) << 16) +
-          (av_toupper((x >> 24) & 0xFF) << 24);
+((unsigned)av_toupper((x >> 24) & 0xFF) << 24);
 }
 
 int ff_thread_ref_frame(ThreadFrame *dst, ThreadFrame *src)