sh(1): fix history file write checking

We cannot just compare histsizeval() against 0, since that returns
a string pointer, which is always non-zero (non-null). The logic
in sethistsize() initializes the history size to 100 with values
that are non-number, and an empty string counts as that. Therefore,
the only time we want to not write into history with HISTSIZE val
set is when it's explicitly 0.

MFC after: 2 weeks