Low: libcrmservice: Fix an error when coverage is enabled.

For some reason that I haven't been able to track down, the following
warning occurs only when you build with coverage enabled:

dbus.c: In function 'handle_query_result':
dbus.c:598:9: error: 'error' may be used uninitialized [-Werror=maybe-uninitialized]

598 |     if (pcmk_dbus_find_error((void*)&error, reply, &error)) {
    |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You don't even have to "make coverage". Just configuring with coverage
enabled and then building normally is enough.