shared/shell: Fix history display with no saved prompt

shared/shell: Fix history display with no saved prompt

Prompt needs to be restored always since default prompt is provided.
This fix issues with history line being clobbered by offset of prompt
length.

[bluetooth]# connect 9C:5C:F9:AB:C5:82
Attempting to connect to 9C:5C:F9:AB:C5:82
[bluetooth]# disconnect 9C:5C:F9:AB:C5:82
Attempting to disconnect from 9C:5C:F9:AB:C5:82
Failed to connect: org.bluez.Error.Failed
Successful disconnected
[bluetooth]# discoverable on
Changing discoverable on succeeded
[CHG] Controller 5C:E0:C5:34:AE:1C Discoverable: yes
[bluetooth]# disconnect discoverable on

after pressing up-down
[bluetooth]# disconnect dis5C:F9:AB:C5:82

Change-Id: I83d4d5e15cb67f51256359d7e73c19a840c85a34
Signed-off-by: Amit Purwar <amit.purwar@samsung.com>