[!ENABLE_ASSERTIONS]: Guard NDEBUG definition.

[!ENABLE_ASSERTIONS]: Guard NDEBUG definition.
(checkfp): Fix off-by-one error that resulted in writing one byte
beyond the end of a malloc'd buffer.  It caused `sort -c' to segfault
on Linux systems having a relatively recent libc.  Before, running
the command, perl -e "print 'x' x 30, \"\n\";"|sort -c
would provoke the memory overrun (though not necessarily the failure).
Add an assertion.