celp filters: Do not read earlier than the start of the 'out' vector.

CC: libav-stable@libav.org
(cherry picked from commit 37ddd38332)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit 9ea94c44b1)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit 08c81f7365)

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

libavcodec/celp_filters.c

@@ -133,9 +133,8 @@ void ff_celp_lp_synthesis_filterf(float *out, const float *filter_coeffs,
         out2 -= val * old_out2;
         out3 -= val * old_out3;
 
-        old_out3 = out[-5];
-
         for (i = 5; i <= filter_length; i += 2) {
+            old_out3 = out[-i];
             val = filter_coeffs[i-1];
 
             out0 -= val * old_out3;
@@ -154,7 +153,6 @@ void ff_celp_lp_synthesis_filterf(float *out, const float *filter_coeffs,
 
             FFSWAP(float, old_out0, old_out2);
             old_out1 = old_out3;
-            old_out3 = out[-i-2];
         }
 
         tmp0 = out0;