fnematch: fix out-of-bounds access on EOF

fnematch: fix out-of-bounds access on EOF

fnematch() expects to store a NUL byte when EOF is encountered.
However, the rewrite broke this assumption because r.len from getrune()
is zero on EOF.  This results in j becoming negative on EOF, causing an
out-of-bounds access.  It is simplest to just force r.len to 1 on EOF
to copy a single NUL byte--the rune is initialized to zero even for EOF.

This also fixes the call to adjbuf().  We cannot use 'k' to determine
when we need to expand the buffer now that we are potentially reading
more than a single byte at a time.

https://github.com/onetrueawk/awk/pull/211