Don't crash after PulseAudio restarts

If PulseAudio daemon gets restarted, wys then crashes when trying to set up call audio afterwards:

wys[5337]: Assertion 'o' failed at pulse/operation.c:67, function pa_operation_unref(). Aborting.

#0  0x0000ffffa9065714 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x0000ffffa90538e8 in __GI_abort () at abort.c:79
#2  0x0000ffffa91dfed0 in pa_operation_unref () at /lib/aarch64-linux-gnu/libpulse.so.0
#3  0x0000ffffa9457d0c in g_cclosure_marshal_VOID__BOOLEANv () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#4  0x0000ffffa9455c5c in  () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#5  0x0000ffffa9472690 in g_signal_emit_valist () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#6  0x0000ffffa9472ffc in g_signal_emit_by_name () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#7  0x0000aaaab5a44b1c in update_direction_state
    (self=0xffff8c012440, mm_call=0xffff880121c0, path=0xaaaad6d3d3c0 "/org/freedesktop/ModemManager1/Call/1", direction=WYS_DIRECTION_FROM_NETWORK, old_state=<optimized out>, new_state=<optimized out>)
    at ../src/wys-modem.c:151
#8  0x0000aaaab5a44c9c in call_state_changed_cb (mm_gdbus_call=<optimized out>, old_state=MM_CALL_STATE_DIALING, new_state=MM_CALL_STATE_RINGING_OUT, reason=<optimized out>, self=0xffff8c012440)
    at ../src/wys-modem.c:183
#9  0x0000ffffa8ed0dcc in ffi_call_SYSV () at /lib/aarch64-linux-gnu/libffi.so.6
#10 0x0000ffffa8ed16f4 in ffi_call () at /lib/aarch64-linux-gnu/libffi.so.6
#11 0x0000ffffa9456338 in g_cclosure_marshal_generic () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#12 0x0000ffffa94559f8 in g_closure_invoke () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#13 0x0000ffffa946a2b8 in  () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#14 0x0000ffffa94718c0 in g_signal_emitv () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#15 0x0000ffffa928da04 in  () at /lib/aarch64-linux-gnu/libmm-glib.so.0
#16 0x0000ffffa8ed0dcc in ffi_call_SYSV () at /lib/aarch64-linux-gnu/libffi.so.6
#17 0x0000ffffa8ed16f4 in ffi_call () at /lib/aarch64-linux-gnu/libffi.so.6
#18 0x0000ffffa9456224 in g_cclosure_marshal_generic () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#19 0x0000ffffa94559f8 in g_closure_invoke () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#20 0x0000ffffa9469b30 in  () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#21 0x0000ffffa947262c in g_signal_emit_valist () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#22 0x0000ffffa9472b98 in g_signal_emit () at /lib/aarch64-linux-gnu/libgobject-2.0.so.0
#23 0x0000ffffa95bfa00 in  () at /lib/aarch64-linux-gnu/libgio-2.0.so.0
#24 0x0000ffffa95ad658 in  () at /lib/aarch64-linux-gnu/libgio-2.0.so.0
#25 0x0000ffffa936151c in g_main_context_dispatch () at /lib/aarch64-linux-gnu/libglib-2.0.so.0
#26 0x0000ffffa93618e8 in  () at /lib/aarch64-linux-gnu/libglib-2.0.so.0
#27 0x0000ffffa9361c80 in g_main_loop_run () at /lib/aarch64-linux-gnu/libglib-2.0.so.0
#28 0x0000aaaab5a439ec in run (modem=<optimized out>) at ../src/main.c:363
#29 0x0000aaaab5a439ec in main (argc=<optimized out>, argv=<optimized out>) at ../src/main.c:642

wys gets restarted by systemd automatically and then manages to set up call audio just fine, so it's not very visible for the user, but we shouldn't crash nevertheless.