Fix null dereference in ldisc_configure.

Fix null dereference in ldisc_configure.

The IDM_RECONF handler unconditionally calls ldisc_configure to
reconfigure the line discipline for the new echo/edit settings, but in
fact ldisc can be NULL if no session is currently active. (Indeed, the
very next line acknowledges this, by testing it for NULL before
calling ldisc_send!) Thanks to Alexander Wong for the report.

[originally from svn r10214]