avcodec/scpr: Fix multiple runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

Fixes: 1422/clusterfuzz-testcase-minimized-5030993939398656

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

libavcodec/scpr.c

@@ -161,7 +161,7 @@ static int get_freq(RangeCoder *rc, unsigned total_freq, unsigned *freq)
 
 static int decode0(GetByteContext *gb, RangeCoder *rc, unsigned cumFreq, unsigned freq, unsigned total_freq)
 {
-    int t;
+    unsigned t;
 
     if (total_freq == 0)
         return AVERROR_INVALIDDATA;