Fix compiler warning

Clang warning: 
'this' pointer cannot be null in well-defined C++ code; comparison may
be assumed to always evaluate to false [-Wtautological-undefined-compare]



git-svn-id: http://sfntly.googlecode.com/svn/trunk/cpp/src@239 672e30a5-4c29-85ac-ac6d-611c735e0a51
diff --git a/sfntly/table/bitmap/bitmap_glyph_info.cc b/sfntly/table/bitmap/bitmap_glyph_info.cc
index ab9953b..a299b3e 100644
--- a/sfntly/table/bitmap/bitmap_glyph_info.cc
+++ b/sfntly/table/bitmap/bitmap_glyph_info.cc
@@ -52,7 +52,7 @@
 
 bool BitmapGlyphInfo::operator==(BitmapGlyphInfo* rhs) {
   if (rhs == NULL) {
-    return this == NULL;
+    return false;  // Well defined C++ code's this is always not null.
   }
   return (format_ == rhs->format() &&
           glyph_id_ == rhs->glyph_id() &&