test ctf writer: fix lints

test ctf writer: fix lints

Coverity found the following defects:

** CID 1124096: Null pointer dereference (FORWARD_NULL)
/tests/lib/test_ctf_writer.c: 140

** CID 1124095: Dereference after null check (FORWARD_NULL)
/tests/lib/test_ctf_writer.c: 139

** CID 1124094: Dereference after null check (FORWARD_NULL)
/tests/lib/test_ctf_writer.c: 211

** CID 1124093: Argument cannot be negative (NEGATIVE_RETURNS)
/tests/lib/test_ctf_writer.c: 144

** CID 1124092: Argument cannot be negative (NEGATIVE_RETURNS)
/tests/lib/test_ctf_writer.c: 144

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