Fix for GrAtlasTextContext crash

src/gpu/GrAtlasTextBlob.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrAtlasTextBlob_DEFINED
 #define GrAtlasTextBlob_DEFINED
 
@@ skipping 81 matching lines @@
             uint32_t fGlyphStartIndex;
             uint32_t fGlyphEndIndex;
             SkScalar fTextRatio; // df property
             GrMaskFormat fMaskFormat;
             bool fDrawAsDistanceFields; // df property
             bool fUseLCDText; // df property
         };
 
         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;
 
             newSubRun.fVertexStartIndex = prevSubRun.fVertexEndIndex;
             newSubRun.fVertexEndIndex = prevSubRun.fVertexEndIndex;
             return newSubRun;
         }
         static const int kMinSubRuns = 1;
         SkAutoTUnref<GrBatchTextStrike> fStrike;
         SkAutoTUnref<SkTypeface> fTypeface;
@@ skipping 106 matching lines @@
     void setHasDistanceField() { fTextType |= kHasDistanceField_TextType; }
     void setHasBitmap() { fTextType |= kHasBitmap_TextType; }
 
 #ifdef CACHE_SANITY_CHECK
     static void AssertEqual(const GrAtlasTextBlob&, const GrAtlasTextBlob&);
     size_t fSize;
 #endif
 };
 
 #endif