Fix problem found by the folks at www.digit-labs.org. Process all the

Fix problem found by the folks at www.digit-labs.org.  Process all the
TLVs in decode_labeled_vpn_l2(), but don't format them unless there's
buffer space left.  If snprintf() returns a negative value, assume we've
filled up the buffer - I think some platforms used to work that way.  If
it returns a value greater than the amount of space left, also assume
we've filled up the buffer.

Fix a typo in a comment.