Fix sign column redraw

Fixed a bug introduced in SHA:aa66f2487edde49b9a5ba10cd70d706d06a94e25,
due to a misapplied patch. buf_del_sign should redraw the window if the
sign deleted was the last one in the buffer.
Also moved the curwin verification to the correct function.
Merge pull request #1311 from GokuITA/issue1295

Fixed sign column redraw