Fix a typo: argv -> argvp

Fix a typo: argv -> argvp

Rationale:
We are interested in the current (last) element of the argv array
there, not in its first element.  The if construct is there because
we want to avoid adding empty (zero-length) arguments to argv, so
we just don't advance argvp if the current argument is empty, and
it gets overwritten at the next iteration.  Note that strsep(3)
doesn't treat consecutive delim characters as a single separator,
it returns empty fields between such characters, and it's up to the
caller to handle them this or that way.

Also add a comment that the argv array ends up null-terminated in
any case (it's due to the design of the for loop) as an answer to
a possible question why the whole argv isn't zero-filled.

Submitted by:	yongari
Tested by:	yongari
MFC after:	3 days