BUG/MINOR: check: test if server is not null in purge

Test if server is not null before using free_server in the check purge
operation. Currently, the null server scenario should not occured as
purge is used with refcounted dynamic servers. However, this might not
be always the case if purge is use in the future in other cases; thus
the test is useful for extensibility.

No need to backport, unless dynamic server checks are backported.

This has been reported through a coverity report in github issue #1343.
diff --git a/src/check.c b/src/check.c
index b49aca6..b1ffe5e 100644
--- a/src/check.c
+++ b/src/check.c
@@ -1253,7 +1253,8 @@
 		task_destroy(check->task);
 		t = NULL;
 
-		free_server(check->server);
+		if (check->server)
+			free_server(check->server);
 	}
 
 	return t;