lineedit: histfile can get emptied when CONFIG_FEATURE_EDITING_SAVE_ON_EXIT=y

lineedit: histfile can get emptied when CONFIG_FEATURE_EDITING_SAVE_ON_EXIT=y

When CONFIG_FEATURE_EDITING_SAVE_ON_EXIT is set to y, the histfile
will get cleared if the total amount of history lines is less than MAX_HISTORY.
Only if the histfile is not empty _and_ the amount of lines currently
in memory are equal to or greater than MAX_HISTORY, history saving will
work as expected with this feature enabled.

Output from defconfig + CONFIG_FEATURE_EDITING_SAVE_ON_EXIT=y:
$ echo "foo" > ~/.ash_history
$ ./busybox ash
~/busybox/a $ echo "bar" > /dev/null
~/busybox/a $ exit
$ cat ~/.ash_history
$

Output with the patch applied and same config as above:
$ echo "foo" > ~/.ash_history
$ ./busybox ash
~/busybox/b $ echo "bar" > /dev/null
~/busybox/b $ exit
$ cat ~/.ash_history
foo
echo "bar" > /dev/null
exit
$

Signed-off-by: Dennis Groenen <tj.groenen at gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>