Bug 890085: don't throw in the currentEngine/defaultEngine setters if the defaultenginename pref is invalid, r=mikedeboer

--HG--
extra : transplant_source : I%18%BA%EB%E8%B1%95V%FA%BF%FA%F5L%14%60%BD%FC%A2DK

toolkit/components/search/nsSearchService.js

@@ -2681,7 +2681,13 @@ SearchService.prototype = {
   get _originalDefaultEngine() {
     let defaultPrefB = Services.prefs.getDefaultBranch(BROWSER_SEARCH_PREF);
     let nsIPLS = Ci.nsIPrefLocalizedString;
-    let defaultEngine = defaultPrefB.getComplexValue("defaultenginename", nsIPLS).data;
+    let defaultEngine;
+    try {
+      defaultEngine = defaultPrefB.getComplexValue("defaultenginename", nsIPLS).data;
+    } catch (ex) {
+      // If the default pref is invalid (e.g. an add-on set it to a bogus value)
+      // getEngineByName will just return null, which is the best we can do.
+    }
     return this.getEngineByName(defaultEngine);
   },