ranlib may break archives at random

[Alexandre Oliva]

If you're unlucky enough, the garbage following map[symbol_count-1]
may pass the test and we'd get one (or more) unexpected
archive_member_file_ptr, that causes the archive to become invalid.
It has happened to me for the past few weeks, while building
libstdc++-v3 on some old version of FreeBSD, and it has just happened
to me again on a different port.  Fixed thusly.  Ok to install?