pgsql: Avoid longjump/vfork warning about line_saved_in_history variable

Log Message:
-----------
Avoid longjump/vfork warning about line_saved_in_history variable by
making it volatile.

Modified Files:
--------------
pgsql/src/bin/psql:
mainloop.c (r1.76 -> r1.77)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/mainloop.c.diff?r1=1.76&r2=1.77)