patch 8.2.1749: Vim9: crash when closure fails in nested function

Problem:    Vim9: crash when closure fails in nested function.
Solution:   Handle function returns before dereferencing remaining closures.
            (closes #7008)

src/testdir/test_vim9_func.vim

@@ -1370,6 +1370,20 @@ def Test_double_closure_fails()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_nested_closure_fails()
+  let lines =<< trim END
+    vim9script
+    def FuncA()
+      FuncB(0)
+    enddef
+    def FuncB(n: number): list<string>
+      return map([0], {_, v -> n})
+    enddef
+    FuncA()
+  END
+  CheckScriptFailure(lines, 'E1012:')
+enddef
+
 def Test_sort_return_type()
   let res: list<number>
   res = [1, 2, 3]->sort()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1749,
 /**/
     1748,
 /**/

src/vim9execute.c

@@ -310,9 +310,12 @@ handle_closure_in_use(ectx_T *ectx, int free_arguments)
     // Check if any created closure is still in use.
     for (idx = 0; idx < closure_count; ++idx)
     {
-	partial_T *pt = ((partial_T **)gap->ga_data)[gap->ga_len
-							- closure_count + idx];
+	partial_T   *pt;
+	int	    off = gap->ga_len - closure_count + idx;
 
+	if (off < 0)
+	    continue;  // count is off or already done
+	pt = ((partial_T **)gap->ga_data)[off];
 	if (pt->pt_refcount > 1)
 	{
 	    int refcount = pt->pt_refcount;
@@ -2734,14 +2737,14 @@ done:
     ret = OK;
 
 failed:
-    // Also deal with closures when failed, they may already be in use
-    // somewhere.
-    handle_closure_in_use(&ectx, FALSE);
-
     // When failed need to unwind the call stack.
     while (ectx.ec_frame_idx != initial_frame_idx)
 	func_return(&ectx);
 
+    // Deal with any remaining closures, they may be in use somewhere.
+    if (ectx.ec_funcrefs.ga_len > 0)
+	handle_closure_in_use(&ectx, FALSE);
+
     estack_pop();
     current_sctx = save_current_sctx;