Only call GetFromExtension if we have an extension. (Bug 523672) r=bzbarsky

uriloader/exthandler/unix/nsMIMEInfoUnix.cpp

@@ -79,9 +79,11 @@ nsMIMEInfoUnix::GetHasDefaultHandler(PRBool *_retval)
   nsRefPtr<nsMIMEInfoBase> mimeInfo = nsGNOMERegistry::GetFromType(mType);
   if (!mimeInfo) {
     nsCAutoString ext;
-    GetPrimaryExtension(ext);
+    nsresult rv = GetPrimaryExtension(ext);
+    if (NS_SUCCEEDED(rv)) {
       mimeInfo = nsGNOMERegistry::GetFromExtension(ext);
     }
+  }
   if (mimeInfo)
     *_retval = PR_TRUE;