Fix dangling pointer after client.close

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## (unreleased)
 
 * Add Ruby 3.0 to the cross compile list
+* Fix segfault when asking if client was dead after closing it. Fixes #519.
 
 ## 2.1.5
 

ext/tiny_tds/client.c

@@ -232,6 +232,7 @@ static void rb_tinytds_client_free(void *ptr) {
     dbloginfree(cwrap->login);
   if (cwrap->client && !cwrap->closed) {
     dbclose(cwrap->client);
+    cwrap->client = NULL;
     cwrap->closed = 1;
     cwrap->userdata->closed = 1;
   }
@@ -263,6 +264,7 @@ static VALUE rb_tinytds_close(VALUE self) {
   GET_CLIENT_WRAPPER(self);
   if (cwrap->client && !cwrap->closed) {
     dbclose(cwrap->client);
+    cwrap->client = NULL;
     cwrap->closed = 1;
     cwrap->userdata->closed = 1;
   }

test/client_test.rb

@@ -16,6 +16,7 @@ class ClientTest < TinyTds::TestCase
       assert @client.close
       assert @client.closed?
       assert !@client.active?
+      assert @client.dead?
       action = lambda { @client.execute('SELECT 1 as [one]').each }
       assert_raise_tinytds_error(action) do |e|
         assert_match %r{closed connection}i, e.message, 'ignore if non-english test run'