Avoid extra trailing comma. r=vlad

gfx/thebes/src/gfxPangoFonts.cpp

@@ -139,6 +139,8 @@ gfxPangoFontGroup::gfxPangoFontGroup (const nsAString& families,
         fixedFamilies.Append(*familyArray[i]);
         fixedFamilies.AppendLiteral(",");
     }
+    if (fixedFamilies.Length() > 0)
+      fixedFamilies.Truncate(fixedFamilies.Length() - 1); // remove final comma
 
     gfxFont *f = new gfxPangoFont(fixedFamilies, this);
     mFonts.push_back(f);