pgsql: Add check for syscache lookup failure in update_relispartition()

Add check for syscache lookup failure in update_relispartition().

Omitted in commit 05b38c7e6 (though it looks like the original blame
belongs to 9e9befac4). A failure is admittedly unlikely, but if it
did happen, SIGSEGV is not the approved method of reporting it.

Per Coverity. Back-patch to v11 where the broken code originated.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/62148c3520b562e518f17134b22120bab0cb113b

Modified Files
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src/backend/commands/indexcmds.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)