settings: avoid assertion for LoadConnections D-Bus method with relative paths

  $ busctl call org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Settings org.freedesktop.NetworkManager.Settings LoadConnections as 1 relative/filename

triggers a g_critical() assertion in nm_utils_file_is_in_path():

  ...
  #3  0x00007ffff7a19e7d in g_return_if_fail_warning
      (log_domain=log_domain@entry=0x55555586c333 "NetworkManager", pretty_function=pretty_function@entry=0x55555586c0a0 <__FUNCTION__.38585> "nm_utils_file_is_in_path", expression=expression@entry=0x55555586c010 "abs_filename && abs_filename[0] == '/'") at ../glib/gmessages.c:2767
  #4  0x00005555555f1128 in nm_utils_file_is_in_path (abs_filename=abs_filename@entry=0x555555b56670 "dfd", abs_path=<optimized out>) at src/NetworkManagerUtils.c:1077
  #5  0x00005555555a4779 in load_connection (config=<optimized out>, filename=0x555555b56670 "dfd") at src/settings/plugins/keyfile/nms-keyfile-plugin.c:522
  #6  0x00005555557ce291 in nm_settings_plugin_load_connection (self=0x5555559fd400 [NMSKeyfilePlugin], filename=0x555555b56670 "dfd") at src/settings/nm-settings-plugin.c:70
  #7  0x000055555559ccdf in impl_settings_load_connections
      (obj=<optimized out>, interface_info=<optimized out>, method_info=<optimized out>, connection=<optimized out>, sender=<optimized out>, invocation=0x7fffe0015ed0 [GDBusMethodInvocation], parameters=<optimized out>) at src/settings/nm-settings.c:1439
  #8  0x00005555555a9bf9 in dbus_vtable_method_call
      (connection=0x5555559b91b0 [GDBusConnection], sender=sender@entry=0x555555b5c360 ":1.32283", object_path=object_path@entry=0x7fffe0019070 "/org/freedesktop/NetworkManager/Settings", interface_name=<optimized out>, interface_name@entry=0x7fffe002aa70 "org.freedesktop.NetworkManager.Settings", method_name=<optimized out>,
      method_name@entry=0x7fffe00276b0 "LoadConnections", parameters=parameters@entry=0x555555c4a690, invocation=0x7fffe0015ed0 [GDBusMethodInvocation], user_data=0x5555559a1a00)
      at src/nm-dbus-manager.c:947
  #9  0x00007ffff7c506c4 in call_in_idle_cb (user_data=user_data@entry=0x7fffe0015ed0) at ../gio/gdbusconnection.c:4874
  #10 0x00007ffff7a0e8eb in g_idle_dispatch (source=source@entry=0x7fffe00208a0, callback=0x7ffff7c50590 <call_in_idle_cb>, user_data=0x7fffe0015ed0) at ../glib/gmain.c:5627
  #11 0x00007ffff7a11fd0 in g_main_dispatch (context=0x555555994d00) at ../glib/gmain.c:3189
  #12 0x00007ffff7a11fd0 in g_main_context_dispatch (context=context@entry=0x555555994d00) at ../glib/gmain.c:3854
  #13 0x00007ffff7a12368 in g_main_context_iterate (context=0x555555994d00, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:3927
  #14 0x00007ffff7a126b3 in g_main_loop_run (loop=0x555555995e60) at ../glib/gmain.c:4123
  #15 0x000055555558a741 in main (argc=<optimized out>, argv=<optimized out>) at src/main.c:444

Filter out relative filenames early.
11 jobs for master in 29 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #292042
allowed to fail
checkpatch

00:00:59

passed #292049
t_debian:9

00:13:32

passed #292051
t_debian:sid

00:14:54

manual #292050
allowed to fail manual
t_debian:testing
passed #292043
t_fedora:28

00:26:21

passed #292044
t_fedora:29

00:29:52

passed #292045
t_fedora:30

00:24:03

manual #292046
allowed to fail manual
t_fedora:rawhide
passed #292047
t_ubuntu:16.04

00:14:45

failed #292048
t_ubuntu:18.04

00:08:14

 
  Deploy
skipped #292052
pages
 
Name Stage Failure
failed
t_ubuntu:18.04 Test
--- stderr ---
/builds/NetworkManager/NetworkManager/tools/run-nm-test.sh: line 235: 74531 Trace/breakpoint trap (core dumped) "${NMTST_DBUS_RUN_SESSION[@]}" "$TEST" "$@"
-------

Full log written to /builds/NetworkManager/NetworkManager/build/meson-logs/testlog.txt
FAILED: meson-test
/usr/local/bin/meson test --no-rebuild --print-errorlogs
ninja: build stopped: subcommand failed.
ERROR: Job failed: exit code 1