This fixes leaks found by ASAN such as:
  GTESTER tests/test-blockjob
=================================================================
==31442==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x7f88483cba38 in __interceptor_calloc (/lib64/libasan.so.4+0xdea38)
    #1 0x7f8845e1bd77 in g_malloc0 ../glib/gmem.c:129
    #2 0x7f8845e1c04b in g_malloc0_n ../glib/gmem.c:360
    #3 0x5584d2732498 in block_job_txn_new /home/elmarco/src/qemu/blockjob.c:172
    #4 0x5584d2739b28 in block_job_create /home/elmarco/src/qemu/blockjob.c:973
    #5 0x5584d270ae31 in mk_job /home/elmarco/src/qemu/tests/test-blockjob.c:34
    #6 0x5584d270b1c1 in do_test_id /home/elmarco/src/qemu/tests/test-blockjob.c:57
    #7 0x5584d270b65c in test_job_ids /home/elmarco/src/qemu/tests/test-blockjob.c:118
    #8 0x7f8845e40b69 in test_case_run ../glib/gtestutils.c:2255
    #9 0x7f8845e40f29 in g_test_run_suite_internal ../glib/gtestutils.c:2339
    #10 0x7f8845e40fd2 in g_test_run_suite_internal ../glib/gtestutils.c:2351
    #11 0x7f8845e411e9 in g_test_run_suite ../glib/gtestutils.c:2426
    #12 0x7f8845e3fe72 in g_test_run ../glib/gtestutils.c:1692
    #13 0x5584d270d6e2 in main /home/elmarco/src/qemu/tests/test-blockjob.c:377
    #14 0x7f8843641f29 in __libc_start_main (/lib64/libc.so.6+0x20f29)

Add an assert to make sure that the job doesn't have associated txn before free().

[Jeff Cody: N.B., used updated patch provided by John Snow]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>