kconsumer: clean-up: initialize ctf_index before populating it

The 'offset' field of the ctf_index is used uninitialized when it is
populated. Its uninitialized value is read and stored back.  This is
not a problem as the offset is populated later-on.

Coveity reports
1405630 Uninitialized scalar variable
The variable will contain an arbitrary value left from earlier computations.
In lttng_kconsumer_read_subbuffer: Use of an uninitialized variable (CWE-457)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>