MDEV-4808: Assertion: trx->start_file != 0 fails in trx0trx.cc on killing CREATE TABLE query.

Analysis: There is debug assertion ut_ad(trx->start_file != 0); and ut_ad(trx->start_line != 0); on trx_start_low funcition at trx0trx.cc. These fields are initialized on include/trx0trx.h at function trx_start_if_not_started_xa. Thus at trx_prepare_for_mysql function should call trx_start_if_not_started_xa(trx); not trx_start_if_not_started_xa_low(trx) directly;

storage/innobase/trx/trx0trx.cc

@@ -1872,7 +1872,7 @@ trx_prepare_for_mysql(
 /*==================*/
 	trx_t*	trx)	/*!< in/out: trx handle */
 {
-	trx_start_if_not_started_xa_low(trx);
+	trx_start_if_not_started_xa(trx);
 
 	trx->op_info = "preparing";