Fix overflow caught by ASAN.

BUG=skia:

Review URL: https://codereview.chromium.org/1498923002
diff --git a/src/codec/SkBmpRLECodec.h b/src/codec/SkBmpRLECodec.h
index fcf910d..df2a97d 100644
--- a/src/codec/SkBmpRLECodec.h
+++ b/src/codec/SkBmpRLECodec.h
@@ -90,6 +90,7 @@
     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;