gitk: Fix Tcl error: can't unset findcurline

The logic in stopfinding assumes that findcurline will be set if
find_dirn is, but findnext and findprev can set find_dirn without
setting findcurline.  This makes sure we only set find_dirn in those
places if findcurline is already set.

Signed-off-by: Paul Mackerras <paulus@samba.org>