Bug 1735233 - Fix unused-but-set-variable warning in MacRunFromDmgUtils.mm. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D128129

toolkit/xre/MacRunFromDmgUtils.mm

@@ -243,6 +243,7 @@ static void ShowInstallFailedDialog() {
   // AskUserIfWeShouldInstall for a detailed explanation.
   [NSApp run];
   MOZ_ASSERT(result != -1);
+  (void)result;
 
   NS_OBJC_END_TRY_IGNORE_BLOCK;
 }