avcodec/lagarith: Fix runtime error: left shift of negative value -1

Fixes: 1424/clusterfuzz-testcase-minimized-6088327159611392

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

libavcodec/lagarith.c

@@ -98,7 +98,7 @@ static uint32_t softfloat_mul(uint32_t x, uint64_t mantissa)
 
 static uint8_t lag_calc_zero_run(int8_t x)
 {
-    return (x << 1) ^ (x >> 7);
+    return (x * 2) ^ (x >> 7);
 }
 
 static int lag_decode_prob(GetBitContext *gb, uint32_t *value)