sbin/hammer: Make strtrl() api right [2/3]

sbin/hammer: Make strtrl() api right [2/3]

- Fix changes made by commit 3c10747d.

- Not sure what the return value of strtrl() is trying to do, but
  checking ==NULL here doesn't mean anything unless 'path' is NULL.
  (The only case that had meaning was when 'path' was NULL, but
  it's pretty obvious callers were not passing NULL)

- Change strnlen() to strlen(). It's better to use strlen() when
  we know the input (argv[i]) is a C string. Also note that in
  this case non NULL terminated char[] of length 'size_t maxlen'
  is useless and does no good anyway.