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

Information object set RANAP-ELEMENTARY-PROCEDURES is a union of
RANAP-ELEMENTARY-PROCEDURES-CLASS-1 and RANAP-ELEMENTARY-PROCEDURES-CLASS-2.

Each time _asn1f_assign_cell_value() parses an object, value of an
incremented counter is assigned to '_type_unique_index' field for
distinquishing different items.

However, these two information object sets have their own counters.
It has chance that generated variables have the same name and also the same
'_type_unique_index'. And they collide if they join up into one set.

S1AP's ASN.1 is lucky without this problem, while RANAP has.

By adding the number of items of the first set to '_type_unique_index' field
of items of the second set can solve this issue.