DESC
Unlock ts->lock if malloc() fails inside jtrans_add().
EDESC
If malloc() fails inside jtrans_add(), we leave ts->lock locked, which will
cause a deadlock if we try to do almost any other operation with the
transaction (in fact, freeing it will also make pthread_mutex_destroy() fail).

This patch fixes it by unlocking before returning in those cases.

Thanks to Shehjar Tikoo for the report.