libsystemd: fix compiler warning in property_get_set_callbacks_run()

gcc (4.8.2, arm) doesn't understand that vtable_property_get_userdata()
will always set 'u' when it returns > 0. Hence, the warning is bogus,
but anyway.

  src/libsystemd/sd-bus/bus-objects.c:510:19: warning: 'u' may be used uninitialized in this function [-Wmaybe-uninitialized]

(and yes, indeed, even the reported line numbers are bogus in this case)
journal: fix compiler warning in real_journal_next()

gcc (4.8.2, arm) does not understand that next_beyond_location() will
always set 'p' when it returns > 0. Initialize p in order to fix this.
journal: fix compiler warning in journal_file_append_data()

gcc (4.8.2, arm) does not understand that journal_file_append_field()
will always set 'fo' when it returns 0, so this warning is bogus.

Anyway, fix it by initialiting fo = NULL.