Fix babeltrace-log uninitialized memory

Got a crash with babeltrace-log.

../../../babeltrace/include/babeltrace/mmap-align.h:78
index=0, whence=0)
    at ../../../babeltrace/formats/ctf/ctf.c:623
    at ../../../babeltrace/formats/ctf/ctf.c:576
<_IO_2_1_stdin_>)
    at ../../babeltrace/converter/babeltrace-log.c:233
../../babeltrace/converter/babeltrace-log.c:342

This issue because pos is used before it init.  This patch init it in
function ctf_init_pos.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>