this patch fixes an issue in jtag_init - shall not reset jtag to NULL.
This causes jtag interface (USB device) not to be closed properly if jtag chain validation fails.

Once the underlaying jtag interface is initialized, jtag pointer shall be left untouched,
even in case of an error, so that proper cleanup can be done, see exit_handler in openocd.c


git-svn-id: svn://svn.berlios.de/openocd/trunk@372 b42882b7-edfa-0310-969c-e2dbd0fdcd60