refspec-api: avoid uninitialized field in refspec item

refspec-api: avoid uninitialized field in refspec item

When parse_refspec() function was created at 3eec3700 ("refspec:
factor out parsing a single refspec", 2018-05-16) to take a caller
supplied piece of memory to fill parsed refspec_item, it forgot that
a refspec without colon must set item->dst to NULL to let the users
of refspec know that the result of the fetch does not get stored in
an ref on our side.

Signed-off-by: Junio C Hamano <redacted>