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

MozReview-Commit-ID: C60kGkoFBL8

--HG--
extra : rebase_source : 3c10097afb406750fe1c2f7410372f3dea4ce52a

modules/libpref/Preferences.cpp

@@ -2895,6 +2895,8 @@ 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;
     }
   }