Fix tuple deforming with virtual generated columns

Fix tuple deforming with virtual generated columns

TupleDescFinalize() failed to take into account virtual generated
columns, which are always stored as NULL in tuples.  TupleDescFinalize()
didn't check for this, and that could result in attcacheoff being set for
and beyond virtual generated columns.  Also, the TupleDesc's
firstNonGuaranteedAttr could also be set incorrectly, which could result
in the tuple deformation function deforming without checking for NULLs,
and deforming using incorrectly cached offsets.

This could result in tuples being deformed incorrectly, which could
result in incorrect results, ERRORs or possibly a crash.

This has been broken since c456e39113.

Author: Chao Li <li.evan.chao@gmail.com>
Reported-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: ChangAo Chen <cca5507@qq.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/A4BC563C-0CA3-4EF3-952A-EA41F9E5BF1E%40gmail.com