Fix overflow caught by ASAN.

src/codec/SkBmpRLECodec.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.
  */
 
 #include "SkBmpCodec.h"
 #include "SkColorTable.h"
 #include "SkImageInfo.h"
@@ skipping 72 matching lines @@
     void setRGBPixel(void* dst, size_t dstRowBytes,
                      const SkImageInfo& dstInfo, uint32_t x, uint32_t y,
                      uint8_t red, uint8_t green, uint8_t blue);
 
     int decodeRows(const SkImageInfo& dstInfo, void* dst, size_t dstRowBytes,
             const Options& opts) override;
 
     SkSampler* getSampler(bool createIfNecessary) override;
 
     SkAutoTUnref<SkColorTable>          fColorTable;    // owned
+    // fNumColors is the number specified in the header, or 0 if not present in the header.
     const uint32_t                      fNumColors;
     const uint32_t                      fBytesPerColor;
     const uint32_t                      fOffset;
     SkAutoTDeleteArray<uint8_t>         fStreamBuffer;
     size_t                              fRLEBytes;
     uint32_t                            fCurrRLEByte;
     int                                 fSampleX;
     SkAutoTDelete<SkSampler>            fSampler;
 
     typedef SkBmpCodec INHERITED;
 };