avcodec/mpegaudiodec_template: Fix 2 runtime error: signed integer overflow

Fixes: 873/clusterfuzz-testcase-5714546230558720

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

libavcodec/mpegaudiodec_template.c

@@ -1038,7 +1038,8 @@ static void compute_stereo(MPADecodeContext *s, GranuleDef *g0, GranuleDef *g1)
 {
     int i, j, k, l;
     int sf_max, sf, len, non_zero_found;
-    INTFLOAT (*is_tab)[16], *tab0, *tab1, tmp0, tmp1, v1, v2;
+    INTFLOAT (*is_tab)[16], *tab0, *tab1, v1, v2;
+    SUINTFLOAT tmp0, tmp1;
     int non_zero_found_short[3];
 
     /* intensity stereo */