setsector: Clean up read beyond end of string

setsector: Clean up read beyond end of string

setsector() reads the first two characters unconditionally.  Wrong if
the first character is NUL.  The second character read isn't actually
used then.  Screwed up in Chainsaw.

When getstarg() returns an empty string, it's always in the buffer
passed as third argument.  Thus, reading the second character is
actually safe.

Clean it up anyway.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>