Fixed access of freed memory in osync_obj_engine_command()
Fixed access of freed memory in osync_obj_engine_command()

The original commit from Daniel Gollub, to add preliminary uni-directional
syncs to opensync can be found at commit
36285e6fce930c42fc6a27789291440d8aab5dbf.

Commit f8dce9758d5bdc10863d107c4cddc44323b86285 fixed some missing code
in one of the functions added in Daniel's commit.

Now that _osync_obj_engine_num_write_sinks() is working, this exposed
a memory bug, where _osync_obj_engine_read_callback() was called outside
of its normal callback sequence, and therefore did not have its userdata
'sinkengine' ref'd properly.  The callback unref'd the memory too early,
causing potential crashes.

I'm still unclear why _osync_obj_engine_read_callback() is being called
here in the first place, but I'll leave that for another commit.

luizluca/opensync-luizluca,luizluca/opensync-luizluca