patch 9.0.0432: crash when using for loop variable in closure

Problem:    Crash when using for loop variable in closure.
Solution:   Check that the variable wasn't deleted. (issue #11094)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 30ff7a7..d4d1ad6 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1716,6 +1716,12 @@
 	return NULL;
     }
     sv = ((svar_T *)si->sn_var_vals.ga_data) + sref->sref_idx;
+    if (sv->sv_name == NULL)
+    {
+	if (dfunc != NULL)
+	    emsg(_(e_script_variable_was_deleted));
+	return NULL;
+    }
     if (!equal_type(sv->sv_type, sref->sref_type, 0))
     {
 	if (dfunc != NULL)