pgsql: Prevent some buffer overruns in spell.c's parsing of affix files

Prevent some buffer overruns in spell.c's parsing of affix files.

parse_affentry() and addCompoundAffixFlagValue() each collect fields
from an affix file into working buffers of size BUFSIZ. They failed
to defend against overlength fields, so that a malicious affix file
could cause a stack smash. BUFSIZ (typically 8K) is certainly way
longer than any reasonable affix field, but let's fix this while
we're closing holes in this area.

I chose to do this by silently truncating the input before it can
overrun the buffer, using logic comparable to the existing logic in
get_nextfield(). Certainly there's at least as good an argument for
raising an error, but for now let's follow the existing precedent.

Reported-by: Igor Stepansky <igor(dot)stepansky(at)orca(dot)security>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/864123.1776810909@sss.pgh.pa.us
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0b196d3db7138967d135b72ed9296a9ad7c06846

Modified Files
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src/backend/tsearch/spell.c | 34 ++++++++++++++++++++++++----------
1 file changed, 24 insertions(+), 10 deletions(-)