lib: don't abort when D-Bus send_with_reply leaves a NULL pending call

[shawnyeager]

Fixes #347.

make_request called dbus_pending_call_block(call) without checking that send_with_reply actually produced a pending call. libdbus documents that dbus_connection_send_with_reply() can return TRUE while leaving *pending_return NULL when the connection is disconnected, or when the message carries unix fds the transport cannot send. Blocking on that NULL handle aborts:

arguments to dbus_pending_call_block() were incorrect, assertion "pending != NULL" failed

That is the gamescopereaper SIGABRT on process exit when libgamemodeauto is in LD_PRELOAD (gamemoderun gamescope ...). gamescopereaper closes every fd except stdio (CloseAllFds) after the GameMode constructor has already opened the session bus. The destructor then calls UnregisterGame:

1. The pidfd attempt still looks connected, so send_with_reply creates a real pending call. The write hits the dead fd, libdbus disconnects, and make_request returns -1.
2. The retry path (use_pidfs = 0) calls make_request again. The connection is now known-disconnected, so send_with_reply leaves call == NULL, and dbus_pending_call_block(NULL) aborts.

Checking only the boolean return is not enough. This change also rejects call == NULL and returns -1.

meson test includes a regression test that closes the session-bus socket without dbus_connection_close() and then calls UnregisterGame. Without the guard it dies with SIGABRT and the assertion above. With the guard it exits 0.

Verified locally:

# without the guard
meson test ... 'client does not abort after the session bus fd is closed'
FAIL  killed by signal 6 SIGABRT

# with the guard
OK    GameMode ERROR: D-Bus error: Could not send D-Bus message