Bug 1554208 - patch 1 - Add missing checks for shared charmap. r=jwatt

This isn't really specific to the FT2 fontlist, it's a general fixup that I noticed while
working on this. (The missing checks aren't crucial, but mean that we might re-read a cmap
when we shouldn't need to.)

Differential Revision: https://phabricator.services.mozilla.com/D36106

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxFontEntry.cpp

@@ -166,9 +166,10 @@ bool gfxFontEntry::TestCharacterMap(uint32_t aCh) {
 nsresult gfxFontEntry::InitializeUVSMap() {
   // mUVSOffset will not be initialized
   // until cmap is initialized.
-  if (!mCharacterMap) {
+  if (!mCharacterMap && !mShmemCharacterMap) {
     ReadCMAP();
-    NS_ASSERTION(mCharacterMap, "failed to initialize character map");
+    NS_ASSERTION(mCharacterMap || mShmemCharacterMap,
+                 "failed to initialize character map");
   }
 
   if (!mUVSOffset) {