Fix: Dereference null return value

Fix: Dereference null return value

Found by Coverity:

2. returned_null: bt_ctf_field_type_variant_get_tag_name returns null
(checked 4 out of 5 times).

3. var_assigned: Assigning: pathstr = null return value from
bt_ctf_field_type_variant_get_tag_name.

CID 1376176 (#1 of 1): Dereference null return value (NULL_RETURNS).
dereference: Dereferencing a pointer that might be null pathstr when
calling pathstr_to_field_path.

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