avcodec/aacdec_template: skip apply_tns() if max_sfb is 0 (from previous header decode failure)

Fixes: NULL pointer dereference
Fixes: 14723/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_LATM_fuzzer-5654612436058112
Fixes: 14724/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_LATM_fuzzer-5712607111020544

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

libavcodec/aacdec_template.c

@@ -2492,6 +2492,9 @@ static void apply_tns(INTFLOAT coef_param[1024], TemporalNoiseShaping *tns,
     INTFLOAT tmp[TNS_MAX_ORDER+1];
     UINTFLOAT *coef = coef_param;
 
+    if(!mmm)
+        return;
+
     for (w = 0; w < ics->num_windows; w++) {
         bottom = ics->num_swb;
         for (filt = 0; filt < tns->n_filt[w]; filt++) {