Bug 1500983 - Fix StartMenuShortcuts installer INI setting always being overridden. r=agashlin

Differential Revision: https://phabricator.services.mozilla.com/D9453

--HG--
extra : moz-landing-system : lando

toolkit/mozapps/installer/windows/nsis/common.nsh

@@ -5092,10 +5092,11 @@
 
             ; We still accept the plural version for backwards compatibility,
             ; but the singular version takes priority.
+            ClearErrors
             ReadINIStr $R8 $R7 "Install" "StartMenuShortcut"
             ${If} $R8 == "false"
               StrCpy $AddStartMenuSC "0"
-            ${Else}
+            ${ElseIfNot} ${Errors}
               StrCpy $AddStartMenuSC "1"
             ${EndIf}