Fix: Added a null pointer check to bt_ctf_field_name

This function is called indirectly when calling __repr__ (print) in the
python bindings on the definition object returned by the
get_top_level_scope method thus resulting in a segmentation fault when
the name attribute is not set.

Fixes #373

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>