avcodec/ac3dec: check bap before use.

Fixes out of array read
Fixes assertion failure
Fixes asan_static-oob_16431c0_8036_rio_bravo_mono_64_spx.ac3

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3dec.c

@@ -497,6 +497,10 @@ static void ac3_decode_transform_coeffs_ch(AC3DecodeContext *s, int ch_index, ma
             break;
         default: /* 6 to 15 */
             /* Shift mantissa and sign-extend it. */
+            if (bap > 15) {
+                av_log(s->avctx, AV_LOG_ERROR, "bap %d is invalid in plain AC-3\n", bap);
+                bap = 15;
+            }
             mantissa = get_sbits(gbc, quantization_tab[bap]);
             mantissa <<= 24 - quantization_tab[bap];
             break;