Add guard to prevent access to null ptr

BUG=aomedia:3534

Change-Id: I15973db83b8a5eae65faac5e7aa6c7bdde372d1f
(cherry picked from commit 7a0e6508d89a0a451d973cec7a6326804b856a0e)

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index ce6176c..878f69f 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -2588,14 +2588,22 @@
     if (cpi->ref_frame_flags & av1_ref_frame_flag_list[GOLDEN_FRAME]) {
       const YV12_BUFFER_CONFIG *const ref =
           get_ref_frame_yv12_buf(cm, GOLDEN_FRAME);
-      if (ref->y_crop_width != cm->width || ref->y_crop_height != cm->height)
+      if (ref == NULL) {
         cpi->ref_frame_flags ^= AOM_GOLD_FLAG;
+      } else {
+        if (ref->y_crop_width != cm->width || ref->y_crop_height != cm->height)
+          cpi->ref_frame_flags ^= AOM_GOLD_FLAG;
+      }
     }
     if (cpi->ref_frame_flags & av1_ref_frame_flag_list[ALTREF_FRAME]) {
       const YV12_BUFFER_CONFIG *const ref =
           get_ref_frame_yv12_buf(cm, ALTREF_FRAME);
-      if (ref->y_crop_width != cm->width || ref->y_crop_height != cm->height)
+      if (ref == NULL) {
         cpi->ref_frame_flags ^= AOM_ALT_FLAG;
+      } else {
+        if (ref->y_crop_width != cm->width || ref->y_crop_height != cm->height)
+          cpi->ref_frame_flags ^= AOM_ALT_FLAG;
+      }
     }
   }