DESC
Fix several things in jtrans_rollback()
EDESC

This patch fixes several small things in jtrans_rollback():

* malloc() return value
* return -1 on error
* remove bogus comments
* free allocated memory
* don't allow more than one nested commit -> rollback

The last one avoids an endless loop that would cause
commit (failed) -> rollback -> commit (failed) -> rollback -> ...

by only allowing commit to call rollback once.