Suppress file output for the gltestthreading config

Bug: skia:
Change-Id: Icd905ea7e60a05bc3903eb85d111dcf73ce2c4dd
Reviewed-on: https://skia-review.googlesource.com/40690
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 1e02d25..1c89ae0 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -1146,7 +1146,7 @@
 
                 if (!FLAGS_writePath.isEmpty()) {
                     const char* ext = task.sink->fileExtension();
-                    if (!FLAGS_dont_write.contains(ext)) {
+                    if (ext && !FLAGS_dont_write.contains(ext)) {
                         if (data->getLength()) {
                             WriteToDisk(task, md5, ext, data, data->getLength(), nullptr);
                             SkASSERT(bitmap.drawsNothing());