shell: fix history memory reclaim

No ring_buf_get_finish() should be performed on ring_buf memory that
wasn't claimed beforehand. Using ring_buf_get() with a NULL destination
does both the claim and the finish part without retrieving anything.

This is especially important with the ring_buffer revanp where the above
is enforced for proper operation.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>