Make ff_psy_preprocess_end() act like av_freep().

Originally committed as revision 23867 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/psymodel.c

@@ -117,11 +117,14 @@ void ff_psy_preprocess(struct FFPsyPreprocessContext *ctx,
 
 av_cold void ff_psy_preprocess_end(struct FFPsyPreprocessContext *ctx)
 {
+    if (ctx) {
     int i;
     ff_iir_filter_free_coeffs(ctx->fcoeffs);
     if (ctx->fstate)
         for (i = 0; i < ctx->avctx->channels; i++)
             ff_iir_filter_free_state(ctx->fstate[i]);
     av_freep(&ctx->fstate);
+    }
+    ctx = NULL;
 }