ash,hush: drop pointer check before calls to show_history

ash,hush: drop pointer check before calls to show_history

show_history() checks that its argument in non-null so there's
no need to repeat the test at call sites.

function                                             old     new   delta
historycmd                                            25      17      -8
builtin_history                                       29      21      -8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-16)             Total: -16 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>