Fix bug in decoder:
When encoding indefinitely & encountering an optional field at the end of
a sequence, right after an IMPLICIT or POINTER template, the decoder
was not propagating the optionalness and the end-of-contents condition
correctly as it hits the end-of-contents octets instead of the optional
field. This is because IMPLICIT and POINTER push TWO states to look
for the next tag, not just one.
(The first state is "afterImplicit" or "afterPointer", the second one
starts with "beforeIdentifier" as usual).
This finally makes decoding envelopedData messages in cmsutil work.
