bug 593296 - correctly release kern table in HB shaper. r=jdaggett a=blocking2.0

gfx/thebes/gfxHarfBuzzShaper.cpp

@@ -83,6 +83,7 @@ gfxHarfBuzzShaper::~gfxHarfBuzzShaper()
 {
     hb_blob_destroy(mCmapTable);
     hb_blob_destroy(mHmtxTable);
+    hb_blob_destroy(mKernTable);
     hb_face_destroy(mHBFace);
 }