MDEV-6481: Yum Upgrade on CentOS 6.5 causes instant

crash of MariaDB/Galera

mysqld crashes during startup when its started with --wsrep-recover
(mysqld_safe.sh). The problem was that during server startup "wsrep"
handle is not initialized for --wsrep-recover and since the condition
to register wsrep hton depended only on 'wsrep_on' global/session
variables (WSREP(thd) macro), it can eventually get registered and
calls to wsrep handle's functions (commit, rollback, etc.) can be
made (SIGSEGV!).

Fixed by adding a check for "wsrep" pointer in WSREP(thd) macro
(added by lp:1367173). Additionally, a check for WSREP(thd)
(instead of checking the availability of wsrep provider) has been
added before invoking wsrep handle's commit & rollback functions.