aacsbr: prevent out of bounds memcpy().

Fixes Libav Bug 195.
Fixes CVE-2012-0850

This doesn't make the code handle sample rate or upsample/downsample
change properly but this is still a good sanity check.

Based on change by Michael Niedermayer.

Signed-off-by: Alex Converse <alex.converse@gmail.com>
(cherry picked from commit 17ce52912f)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/aacsbr.c

@@ -1181,14 +1181,15 @@ static void sbr_qmf_synthesis(DSPContext *dsp, FFTContext *mdct,
 {
     int i, n;
     const float *sbr_qmf_window = div ? sbr_qmf_window_ds : sbr_qmf_window_us;
+    const int step = 128 >> div;
     float *v;
     for (i = 0; i < 32; i++) {
-        if (*v_off == 0) {
+        if (*v_off < step) {
             int saved_samples = (1280 - 128) >> div;
             memcpy(&v0[SBR_SYNTHESIS_BUF_SIZE - saved_samples], v0, saved_samples * sizeof(float));
-            *v_off = SBR_SYNTHESIS_BUF_SIZE - saved_samples - (128 >> div);
+            *v_off = SBR_SYNTHESIS_BUF_SIZE - saved_samples - step;
         } else {
-            *v_off -= 128 >> div;
+            *v_off -= step;
         }
         v = v0 + *v_off;
         if (div) {