hush: in some cases, expand_on_ifs() relied of uninitialized memory

hush: in some cases, expand_on_ifs() relied of uninitialized memory

The n > 0 check to prevent access to the last byte of non-existing argv[-1]
wasn't enough. Switched to making sure there are initialized (zero) bytes there.

A predictable testcase is rather hard to construct, unfortunately,
contents of memory depends on allocator behavior and whatnot.

function                                             old     new   delta
o_save_ptr_helper                                    119     137     +18
expand_on_ifs                                        345     339      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 18/-6)              Total: 12 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>