Fix memory-safety bugs in the ispell/hunspell dictionary loader.

Fix memory-safety bugs in the ispell/hunspell dictionary loader.

Allocate CompoundAffix with room for its terminator, initialize the
old-format flag buffer before NIAddAffix(), and reject incomplete or
missing Hunspell AF aliases.  None of these errors would be likely to
trigger on real dictionary files, accounting for the lack of previous
reports; but they're certainly bugs.

Bug: #19595
Reported-by: Michael Malis <michaelmalis2@gmail.com>
Author: Andrey Rachitskiy <pl0h0yp1@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19595-7dc18b4e212c4757@postgresql.org
Backpatch-through: 14