ns-do-applescript: protect source code string from garbage collection

As an attempt to fix the crash in ns_do_applescript (TTermEntry::NextSubTerm).
The reasoning here is that the NSString used to construct the AppleScript object
is sufficiently protected from Cocoa's/ObjC GC.  So, perhaps the NS run loop called up
in ns-do-applescript causes Emacs/Lisp to be executed via some callbacks, and its GC.
(This may be a bug in itself.)

src/nsfns.m

@@ -2673,6 +2673,8 @@ and GNUstep implementations ("distributor-specific release
     (script)
     Lisp_Object script;
 {
+  struct gcpro gcpro1;
+
   Lisp_Object result;
   long status;
   NSEvent *nxev;
@@ -2681,6 +2683,7 @@ and GNUstep implementations ("distributor-specific release
   check_ns ();
 
   BLOCK_INPUT;
+  GCPRO1 (script);
 
   as_script = script;
   as_result = &result;
@@ -2706,7 +2709,10 @@ and GNUstep implementations ("distributor-specific release
   as_status = 0;
   as_script = Qnil;
   as_result = 0;
+
+  UNGCPRO;
   UNBLOCK_INPUT;
+
   if (status == 0)
     return result;
   else if (!STRINGP (result))