[lldb/Bindings] Check that process isn't None before calling is_alive.

Make sure that `process` is not None before calling is_alive. Otherwise
this might result in an AttributeError: 'NoneType' object has no
attribute 'is_alive'.

Although lldb.process and friends could already be None in the past, for
example after leaving an interactive scripting session, the issue became
more prevalent after `fc1fd6bf`.

I audited the other interface files for usages of target, process,
thread and frame, but this seems the only place where a global is used
from an SB class.