avcodec/gsmdec_template: Fix runtime error: signed integer overflow: -22527 * 99113 cannot be represented in type 'int'

Fixes: 636/clusterfuzz-testcase-6520876646268928
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/gsmdec_template.c

@@ -41,7 +41,7 @@ static void apcm_dequant_add(GetBitContext *gb, int16_t *dst, const int *frame_b
 
 static inline int gsm_mult(int a, int b)
 {
-    return (a * b + (1 << 14)) >> 15;
+    return (int)(a * (SUINT)b + (1 << 14)) >> 15;
 }
 
 static void long_term_synth(int16_t *dst, int lag, int gain_idx)