Correct error checks in EVP_read_pw_string_min(): UI_add_input_string()

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@