Fix nocachegetattr() so it again supports deforming cstrings

Fix nocachegetattr() so it again supports deforming cstrings

c456e3911 added various optimizations to the tuple deformation routines.
One optimization assumed that heap tuples would never contain cstrings.
That optimization also made its way into nocachegetattr(), which isn't
correct as ROW() types get formed into HeapTuples by ExecEvalRow() and
those can contain cstring Datums.  nocachegetattr() gets used to extract
Datums from those tuples.

Here we remove the pg_assume(), which was there to instruct the compiler
to omit the attlen == -2 related code in att_addlength_pointer().

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/80aeac57-8f50-4732-a5b4-c2373c3f8149@gmail.com