Permit null target on TCL connection

Permit null target on TCL connection

In previous versions of OpenOCD, it was possible to connect to the TCL
RPC interface without a current target. In `tcl_new_connection`, the
curent target would be queried by number, and the possibility of a null
current target was handled properly.

In commit bb9d9c60264a905926e0d15f84842858d0de80b7, the
`get_target_by_num` call was replaced by a `get_current_target` call,
without noticing that `get_current_target` aborts if there is no current
target, whereas `tcl_new_connection` is perfectly able to handle that
situation.

Provide a `get_current_target_or_null` function for use by consumers who
are OK with a null current target, and use it in `tcl_new_connection`.

Change-Id: I06f7e1e149f1169e23c73ba328c7ad9f9425cc2a
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4730
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Franck Jullien <franck.jullien@gmail.com>