write_features_to_file: check fopen() result

This is a follow up to:
dadc6023d ExternalPartitionTestDfsAPI: skip check w/WRITE_FEATURE_TO_FILE=0

Return if the file open fails to avoid a crash. In the case of
WRITE_FEATURE_TO_FILE=0 the test will pass as expected. With
WRITE_FEATURE_TO_FILE=1 this would cause test failure as the comparison
will be performed which is also expected.

Bug: b/191266788
Change-Id: I7fa52b5599533843f6d17c22970583e664a217be
diff --git a/av1/encoder/partition_strategy.c b/av1/encoder/partition_strategy.c
index 269bbfd..91d1a74 100644
--- a/av1/encoder/partition_strategy.c
+++ b/av1/encoder/partition_strategy.c
@@ -115,6 +115,7 @@
   snprintf(filename, sizeof(filename), "%s/%s", path,
            get_feature_file_name(id));
   FILE *pfile = fopen(filename, "a");
+  if (pfile == NULL) return;
   if (!is_test_mode) {
     fprintf(pfile, "%d,%d,%d,%d,%d\n", id, bsize, mi_row, mi_col, feature_size);
   }