Change fpLookup() to return malloced memory (on first call)

Change fpLookup() to return malloced memory (on first call)

- If the fingerprint pointer passed to it is NULL then allocate space
  for a new fingerprint, otherwise reuse the previous space. This should
  allow optimizing the case where repeatedly calling and directory
  doesn't change inside fpc so callers dont need special-case code
  for this. For now, we dont care about optimizations, other than
  making it possible later.