pgsql: Tighten up TS dictionary cache entry creation.

Tighten up TS dictionary cache entry creation.

In the not-too-likely scenario where we successfully created a hash
table entry for a TS dictionary, but then failed to make a small
memory context for it, we left the hash entry in existence but with
a garbage value for dictCtx. This confused the code the next time
through, leading to a crash. Rearrange things so that we leave
the hash entry in a well-defined state with dictCtx == NULL, and
then the next try knows it still needs to make a memory context.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/0f3ddeb5-0dbd-479c-9d0e-ae254758e624@gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/83336e3ed30b0d0b021900429e5bfde8367e2729

Modified Files
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src/backend/utils/cache/ts_cache.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)