avcodec/eac3dec: Fix runtime error: left shift of negative value -3

Fixes: 672/clusterfuzz-testcase-5595018867769344

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 87eb374970)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/eac3dec.c

@@ -280,7 +280,7 @@ static void ff_eac3_decode_transform_coeffs_aht_ch(AC3DecodeContext *s, int ch)
                     mant += ((ff_eac3_gaq_remap_2_4_a[hebap-8][log_gain-1] * (int64_t)mant) >> 15) + b;
                 } else {
                     /* small mantissa, no GAQ, or Gk=1 */
-                    mant <<= 24 - bits;
+                    mant *= (1 << 24 - bits);
                     if (!log_gain) {
                         /* remap mantissa value for no GAQ or Gk=1 */
                         mant += (ff_eac3_gaq_remap_1[hebap-8] * (int64_t)mant) >> 15;