avcodec/ffv1dec: Fix runtime error: signed integer overflow: 1550964438 + 1550964438 cannot be represented in type 'int'

Fixes: 1559/clusterfuzz-testcase-minimized-5048096079740928
Fixes: 1560/clusterfuzz-testcase-minimized-6011037813833728

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

libavcodec/ffv1dec.c

@@ -45,7 +45,8 @@ static inline av_flatten int get_symbol_inline(RangeCoder *c, uint8_t *state,
     if (get_rac(c, state + 0))
         return 0;
     else {
-        int i, e, a;
+        int i, e;
+        unsigned a;
         e = 0;
         while (get_rac(c, state + 1 + FFMIN(e, 9))) { // 1..10
             e++;