encoder_destroy: check ppi->cpi before dereferencing

Fixes a NULL pointer deference if `ppi` is only partially constructed
before hitting an error during initialization.

Bug: 503326506
Fixed: 503326506
Change-Id: I867a46ab9769c6d83fb114bce645303fdd41268c

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index e1e8ef1..b43c96d 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -3233,7 +3233,9 @@
 
   if (ctx->ppi) {
     AV1_PRIMARY *ppi = ctx->ppi;
-    av1_extrc_delete(&ppi->cpi->ext_ratectrl);
+    if (ppi->cpi) {
+      av1_extrc_delete(&ppi->cpi->ext_ratectrl);
+    }
     for (int i = 0; i < MAX_PARALLEL_FRAMES - 1; i++) {
       if (ppi->parallel_frames_data[i].cx_data) {
         free(ppi->parallel_frames_data[i].cx_data);