constr_SET_OF.c:315:13: runtime error: null pointer passed as argument 1, which is declared to never be null

Also move some asserts into dedicated function because b->buf could be
NULL while a->length is greater then zero

A length determinant was not added when the SEQUENCE OF was empty
according to the X.691 #20.6 and #11.9.4.1.

This fixes a failure in 16-SEQUENCE-bundle

Decoding 1 bytes of T yielded RC_WMORE after byte 0
Original random structure:
T ::= {
    list: list ::= {
    }
    guard: 01
}

when the 32bit build is dislabed.

On MacOS with Xcode 9.0 the make check fails because the ar
cannot build empty static libraries

Fixes #228