MEDIUM: checks: exclusively use cs_destroy() to release a connection

This way we're using the more consistent API everywhere.
diff --git a/src/checks.c b/src/checks.c
index 49d9467..0b72bb2 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -2210,7 +2210,7 @@
 			cs_close(cs);
 		}
 
-		if (conn) {
+		if (cs) {
 			cs_destroy(cs);
 			cs = check->cs = NULL;
 			conn = NULL;
@@ -2706,11 +2706,8 @@
 				return retcode;
 			}
 
-			if (check->cs) {
-				if (check->cs->conn)
-					conn_free(check->cs->conn);
-				cs_free(check->cs);
-			}
+			if (check->cs)
+				cs_destroy(check->cs);
 
 			check->cs = cs;
 			conn = cs->conn;