Fix for GrAtlasTextContext crash

src/gpu/GrAtlasTextBlob.h

Index: src/gpu/GrAtlasTextBlob.h
diff --git a/src/gpu/GrAtlasTextBlob.h b/src/gpu/GrAtlasTextBlob.h
index 1f37c676e2cf53303f9b370619719db195192173..502177d0d23247f4cc7fe18810d8fe210e7b0506 100644
--- a/src/gpu/GrAtlasTextBlob.h
+++ b/src/gpu/GrAtlasTextBlob.h
@@ -99,8 +99,9 @@ struct GrAtlasTextBlob : public SkRefCnt {
 
         SubRunInfo& push_back() {
             // Forward glyph / vertex information to seed the new sub run
-            SubRunInfo& prevSubRun = fSubRunInfo.back();
             SubRunInfo& newSubRun = fSubRunInfo.push_back();
+            SubRunInfo& prevSubRun = fSubRunInfo.fromBack(1);
+
             newSubRun.fGlyphStartIndex = prevSubRun.fGlyphEndIndex;
             newSubRun.fGlyphEndIndex = prevSubRun.fGlyphEndIndex;