devicemonitor: Avoid maybe-uninitialized compiler warning

On Arch Linux x86_64, gcc 7.2.0-3, -Og -g3:

gstdevicemonitor.c: In function ‘bus_sync_message’:
gstdevicemonitor.c:276:8: error: ‘matches’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

This commit also simplifies the code a bit.

https://bugzilla.gnome.org/show_bug.cgi?id=789983
1 file changed