drm: Fix crash when terminal fd is still -1 after reconnect

drm: Fix crash when terminal fd is still -1 after reconnect

The drm plugin code installs a fd disconnect handler for the terminal fd
which simply calls open_input_source () again.

This assumes that the ply-terminal code's disconnect handler has run first
(which it should) and that ply_terminal_reopen_device () has successfully
re-opened the terminal. This last condition is not always true, resulting
in open_input_source () calling ply_event_loop_watch_fd () with a -1 fd
triggerig an assert in ply_event_loop_watch_fd ():

5  0x00007f62d39a1c6f in __assert_fail (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at assert.c:127
6  0x00007f62d3bc22c7 in ply_event_loop_watch_fd (loop=<optimized out>, fd=-1, status=status@entry=PLY_EVENT_LOOP_FD_STATUS_HAS_DATA,
    status_met_handler=status_met_handler@entry=0x7f62d3790870 <on_terminal_key_event>,
    disconnected_handler=disconnected_handler@entry=0x7f62d3790c70 <on_input_source_disconnected>, user_data=user_data@entry=0x5647f7dd9fb8)
    at ../src/libply/ply-event-loop.c:732
7  0x00007f62d3790bf6 in open_input_source (backend=0x5647f7dd9f90, input_source=0x5647f7dd9fb8) at ../src/plugins/renderers/drm/plugin.c:1930
8  0x00007f62d3bcbd53 in ply_event_loop_handle_disconnect_for_source (loop=<optimized out>, source=0x5647f7dd69f0) at ../src/libply/ply-event-loop.c:1065
9  ply_event_loop_disconnect_source (loop=<optimized out>, source=0x5647f7dd69f0) at ../src/libply/ply-event-loop.c:1157
10 ply_event_loop_process_pending_events (loop=0x5647f7dd13e0) at ../src/libply/ply-event-loop.c:1277
11 0x00007f62d3bcc068 in ply_event_loop_run (loop=0x5647f7dd13e0) at ../src/libply/ply-event-loop.c:1311
12 0x00005647c99bba48 in main (argc=<optimized out>, argv=<optimized out>) at ../src/main.c:2572

Fix this by checking that the fd >= 0 before calling
ply_event_loop_watch_fd ().

The above backtrace is from the drm plugin, but the same problem exists
in the frame-buffer plugin. So this fix is applied to both.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2370979
Signed-off-by: Hans de Goede <hansg@kernel.org>