crash in write_history_n

[Bastian M. <bma...@we...>]

The version of write_history_n() using gnuplot's own version
of readline issues an int_error() when it cannot open the
history file for saving. As it is now called at program exit
(via atexit() or similar) it can no longer
bail_to_command_line(), which int_error() does. This causes
a segfault in that case. (Tested ;-) on a Win98 system without
%GNUPLOT% env. variable set.)
So this should be an int_warn(). The attached patch fixes that.

Greetings, Bastian