Do not register signs to be placed that have no line number
Do not register signs to be placed that have no line number

This might indirectly help a bit with
https://github.com/neomake/neomake/issues/240, in case signs will be
never processed, but always retried.

neomake/neomake,euclio/neomake,bonds/neomake,nerdrew/neomake,neomake/neomake,euclio/neomake,codeinabox/neomake,datanoise/neomake,bonds/neomake,nerdrew/neomake