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 <gitster@pobox.com>

refspec.c

@@ -48,6 +48,8 @@ static int parse_refspec(struct refspec_item *item, const char *refspec, int fet
 		size_t rlen = strlen(++rhs);
 		is_glob = (1 <= rlen && strchr(rhs, '*'));
 		item->dst = xstrndup(rhs, rlen);
+	} else {
+		item->dst = NULL;
 	}
 
 	llen = (rhs ? (rhs - lhs - 1) : strlen(lhs));