pgsql: Fix corner case bug in numeric to_char() some more.

Fix corner case bug in numeric to_char() some more.

The band-aid applied in commit f0bedf3e4 turns out to still need
some work: it made sure we didn't set Np->last_relevant too small
(to the left of the decimal point), but it didn't prevent setting
it too large (off the end of the partially-converted string).
This could result in fetching data beyond the end of the allocated
space, which with very bad luck could cause a SIGSEGV, though
I don't see any hazard of interesting memory disclosure.

Per bug #17839 from Thiago Nunes. The bug's pretty ancient,
so back-patch to all supported versions.

Discussion: https://postgr.es/m/17839-aada50db24d7b0da@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a67c75f8258742945f4f3703126a1222adeeb85b

Modified Files
--------------
src/backend/utils/adt/formatting.c | 11 +++++++++--
src/test/regress/expected/numeric.out | 6 ++++++
src/test/regress/sql/numeric.sql | 1 +
3 files changed, 16 insertions(+), 2 deletions(-)