avcodec/aac/aacdec_usac: Test ac in usac

ff_aac_usac_config_decode() needs AACDecContext to be set but some callers
pass NULL.

Happens only when the LATM decoder is used, and USAC is not supported in
LATM

Fixes: member access within null pointer of type 'AACDecContext' (aka 'struct AACDecContext')
Fixes: 69435/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_LATM_fuzzer-5733527483121664

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

libavcodec/aac/aacdec_usac.c

@@ -348,6 +348,9 @@ int ff_aac_usac_config_decode(AACDecContext *ac, AVCodecContext *avctx,
     int map_pos_set = 0;
     uint8_t layout_map[MAX_ELEM_ID*4][3] = { 0 };
 
+    if (!ac)
+        return AVERROR_PATCHWELCOME;
+
     memset(usac, 0, sizeof(*usac));
 
     freq_idx = get_bits(gb, 5); /* usacSamplingFrequencyIndex */