fdwrite.c: initialize pointers to NULL and a few other cleanups

fdwrite.c: initialize pointers to NULL and a few other cleanups

1. Both trackbuf and vrfybuf are initialized to
zero (NULL). While it's okay to initialize pointers
to zero, to keep consistency, as they're explicitly
pointers, it's better to just use NULL ((void *)0)
instead of 0 (both are equivalent to the compilers).

2. Call free() for both trackbuf and vrfybuf after
their job has been done.

3. Remove the register keyword. Compilers generally
ignore this keyword (except for very very old compilers
and CPUs).

4. Remove the ctype.h header. It's not being used
anywhere in the file.

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1059