patch 9.1.1879: Crash when using a lambda funcref with :defer

Problem:  Crash when using a lambda funcref with :defer
Solution: De-reference the partial correctly after invoking the deferred
          functions (Yegappan Lakshmanan).

closes: #18640

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: yegappan

src/testdir/test_vim9_script.vim

@@ -5232,6 +5232,23 @@ def Test_defer_invalid_func_arg()
   v9.CheckScriptFailure(lines, 'E1001: Variable not found: a', 1)
 enddef
 
+" Test for using defer with a lambda funcref
+def Test_defer_lambda_funcref()
+  var lines =<< trim END
+    vim9script
+    var lfr_result = ''
+    def Foo()
+      var Fn = () => {
+          lfr_result = 'called'
+        }
+      defer Fn()
+    enddef
+    Foo()
+    assert_equal('called', lfr_result)
+  END
+  v9.CheckSourceSuccess(lines)
+enddef
+
 " Test for using an non-existing type in a "for" statement.
 def Test_invalid_type_in_for()
   var lines =<< trim END

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1879,
 /**/
     1878,
 /**/

src/vim9execute.c

@@ -1176,20 +1176,30 @@ invoke_defer_funcs(ectx_T *ectx)
 	    argvars[i] = arg_li->li_tv;
 	}
 
-	funcexe_T   funcexe;
+	funcexe_T	funcexe;
+	char_u		*name = NULL;
+	partial_T	*pt = NULL;
+
 	CLEAR_FIELD(funcexe);
 	funcexe.fe_evaluate = TRUE;
 	rettv.v_type = VAR_UNKNOWN;
+
 	if (functv->v_type == VAR_PARTIAL)
 	{
-	    funcexe.fe_partial = functv->vval.v_partial;
-	    funcexe.fe_object = functv->vval.v_partial->pt_obj;
+	    pt = functv->vval.v_partial;
+	    functv->vval.v_partial = NULL;
+
+	    name = pt->pt_func->uf_name;
+	    funcexe.fe_partial = pt;
+	    funcexe.fe_object = pt->pt_obj;
 	    if (funcexe.fe_object != NULL)
 		++funcexe.fe_object->obj_refcount;
 	}
-
-	char_u *name = functv->vval.v_string;
-	functv->vval.v_string = NULL;
+	else
+	{
+	    name = functv->vval.v_string;
+	    functv->vval.v_string = NULL;
+	}
 
 	// If the deferred function is called after an exception, then only the
 	// first statement in the function will be executed (because of the
@@ -1204,7 +1214,10 @@ invoke_defer_funcs(ectx_T *ectx)
 	exception_state_restore(&estate);
 
 	clear_tv(&rettv);
-	vim_free(name);
+	if (functv->v_type == VAR_PARTIAL)
+	    partial_unref(pt);
+	else
+	    vim_free(name);
     }
 }