avutil/avstring: Fix undefined shift

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

libavutil/avstring.c

@@ -408,7 +408,7 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
         goto end;
     }
 
-    if (code >= 1<<31) {
+    if (code >= 1U<<31) {
         ret = AVERROR(EILSEQ);  /* out-of-range value */
         goto end;
     }