Fix SkDrawTextBlobCommand null blob crash

After https://codereview.chromium.org/2335493005, we're moving the param
into a local field -> we should no longer access it.

TBR=robertphillips@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2344523003

Review-Url: https://codereview.chromium.org/2344523003
diff --git a/tools/debugger/SkDrawCommand.cpp b/tools/debugger/SkDrawCommand.cpp
index 0f81013..75978d4 100644
--- a/tools/debugger/SkDrawCommand.cpp
+++ b/tools/debugger/SkDrawCommand.cpp
@@ -2813,7 +2813,7 @@
 
     unsigned runs = 0;
     SkPaint runPaint(paint);
-    SkTextBlobRunIterator iter(blob.get());
+    SkTextBlobRunIterator iter(fBlob.get());
     while (!iter.done()) {
         SkAutoTDelete<SkString> tmpStr(new SkString);
         tmpStr->printf("==== Run [%d] ====", runs++);