sbr_qmf_analysis: sanitize input for 32-bit imdct

If the input contains too many too large values, the imdct can overflow.
Even if it didn't, the output would be larger than the valid range of 29
bits.

Note that this is a very delicate limit: Allowing values up to 1<<25
does not prevent input larger than 1<<29 from arriving at
sbr_sum_square, while limiting values to 1<<23 breaks the
fate-aac-fixed-al_sbr_hq_cm_48_5.1 test.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/aacsbr_template.c

@@ -1163,6 +1163,9 @@ static void sbr_qmf_analysis(AVFloatDSPContext *dsp, FFTContext *mdct,
                              INTFLOAT z[320], INTFLOAT W[2][32][32][2], int buf_idx)
 {
     int i;
+#if USE_FIXED
+    int j;
+#endif
     memcpy(x    , x+1024, (320-32)*sizeof(x[0]));
     memcpy(x+288, in,         1024*sizeof(x[0]));
     for (i = 0; i < 32; i++) { // numTimeSlots*RATE = 16*2 as 960 sample frames
@@ -1170,6 +1173,21 @@ static void sbr_qmf_analysis(AVFloatDSPContext *dsp, FFTContext *mdct,
         dsp->vector_fmul_reverse(z, sbr_qmf_window_ds, x, 320);
         sbrdsp->sum64x5(z);
         sbrdsp->qmf_pre_shuffle(z);
+#if USE_FIXED
+        for (j = 64; j < 128; j++) {
+            if (z[j] > 1<<24) {
+                av_log(NULL, AV_LOG_WARNING,
+                       "sbr_qmf_analysis: value %09d too large, setting to %09d\n",
+                       z[j], 1<<24);
+                z[j] = 1<<24;
+            } else if (z[j] < -(1<<24)) {
+                av_log(NULL, AV_LOG_WARNING,
+                       "sbr_qmf_analysis: value %09d too small, setting to %09d\n",
+                       z[j], -(1<<24));
+                z[j] = -(1<<24);
+            }
+        }
+#endif
         mdct->imdct_half(mdct, z, z+64);
         sbrdsp->qmf_post_shuffle(W[buf_idx][i], z);
         x += 32;