Restore EVP_CIPH_FLAG_LENGTH_BITS working properly

EVP_CIPH_FLAG_LENGTH_BITS flag for CFB1 has been broken with the
introduction of the is_partially_overlapping() check that did not take
it into the account (treating number of bits passed as bytes). This
remedies that and allows this flag to work as intended.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1942)