Ensure that *netreg_data is always set in callback data (LP: 1234491).

ril_register_auto() correctly set cdb->user, whereas
ril_register_manual() did not.  As both shared the same callback
function, ril_register_cb(), and this function always expected
cdb->user to be non-NULL, calls to register_manual() could
cause a segfault.
Merge pull request #6 from tonyespy/netreg-crash-fix

Ensure that *netreg_data is always set in callback data (LP: 1234491).