stand: Minor cleanup

Replace a cast '0' for a null pointers with NULL
Replace a 'goto loop' with a do-while loop in ufs and ext2fs.
Cast cp pointer to uintptr_t to test to see if it's aligned rather than long.

[ minor tweaks based on my & hps' review, reworded commit message ]
Reviewed by: imp, hps
Pull Request: https://github.com/freebsd/freebsd-src/pull/547