Remove null checks on g_new0

Remove null checks on g_new0

g_new0 never returns NULL. We need to either always check for NULL, or
never. Remove all NULL checks on pointers returned by g_new0. Coverity
was complaining because we were dereferencing a pointer in an error path
after checking it against NULL, which is certainly never OK.

Found by coverity:

** CID 1136988:  Dereference after null check  (FORWARD_NULL)
/formats/lttng-live/lttng-live.c: 152 in lttng_live_open_trace_read()

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