Bug 1475836: Clear cached callback pref when deleting branch. r=njn

MozReview-Commit-ID: C60kGkoFBL8

--HG--
extra : source : 3b9846062755b05de6729a84aa2b0acf6ddfd0bc

modules/libpref/Preferences.cpp

@@ -2896,6 +2896,8 @@ nsPrefBranch::DeleteBranch(const char* aStartingAt)
     nsDependentCString name(pref->Name());
     if (StringBeginsWith(name, branchName) || name.Equals(branchNameNoDot)) {
       iter.Remove();
+      // The saved callback pref may be invalid now.
+      gCallbackPref = nullptr;
     }
   }