BUG/MEDIUM: dumpstats: undefined behavior in stats_tlskeys_list()

BUG/MEDIUM: dumpstats: undefined behavior in stats_tlskeys_list()

The function stats_tlskeys_list() can meet an undefined behavior when
called with appctx->st2 == STAT_ST_LIST, indeed the ref pointer is used
uninitialized.

However this function was using NULL in appctx->ctx.tlskeys.ref as a
flag to dump every tickets from every references.  A real flag
appctx->ctx.tlskeys.dump_all is now used for this behavior.

This patch delete the 'ref' variable and use appctx->ctx.tlskeys.ref
directly.