fix: short files in PasswordHashChecker

fix: short files in PasswordHashChecker

For short files (or, presumably, for very rare hashes on all files),
PasswordHashChecker would occasionally attempt to read before the start
or past the end of a file; avoid this with clamping (in two cases where
there is no potentially infinite iteration) or aborting (in the one
other case, where clamping might yield an infinite loop).

Change-Id: Ia1d4f527a2b8589ec43732e0e1a1cf80cb3e2bac