Correct error checks in EVP_read_pw_string_min(): UI_add_input_string()
and UI_add_verify_string() return -1 (and maybe -2?) on failure and
>=0 on success, instead of always zero on success

problem reported by Mark Patruck (mark (at) wrapped.cx)
ok miod@
Allow B64_EOF to follow a base64 padding character. This restores previous
behaviour that allows a PEM block to be fed through the base64 decoder.

Reported by Dmitry Eremin-Solenikov on tech@

ok deraadt@ tedu@