Rewrite EVP_DecodeUpdate.

In particular: reject extra trailing padding, and padding in the middle
of the content. Don't limit line length. Add tests.

Previously, the behaviour was ill-defined, and depended on the position
of the padding within the input.

In addition, this appears to fix a possible two-byte oob read.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NDr Stephen Henson <steve@openssl.org>