avcodec/mss3: Change types in rac_get_model_sym() to match the types they are initialized from

Fixes integer overflow
Fixes: 1372/clusterfuzz-testcase-minimized-5712192982745088

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

libavcodec/mss3.c

@@ -356,8 +356,9 @@ static int rac_get_model2_sym(RangeCoder *c, Model2 *m)
 
 static int rac_get_model_sym(RangeCoder *c, Model *m)
 {
-    int prob, prob2, helper, val;
+    int val;
     int end, end2;
+    unsigned prob, prob2, helper;
 
     prob       = 0;
     prob2      = c->range;