asn1: check overflow against SIZE_MAX not +1

A comparison of (len > len + 1) is permitted to be optimized out
as dead code because it can't be true.  Overflowing is an exceptional
condition that results in undefined behavior.  The correct conditional
is (len == SIZE_MAX) when len is size_t.

Change-Id: Ia5586556a973d9fa5228430c4304ea9792c996bb