pgsql: Fix crash when reporting CREATE INDEX progress

Fix crash when reporting CREATE INDEX progress

A race condition can make us try to dereference a NULL pointer to the
PGPROC struct of a process that's already finished.  That results in
crashes during REINDEX CONCURRENTLY and CREATE INDEX CONCURRENTLY.

This was introduced in ab0dfc961b6a, so backpatch to pg12.

Reported by: Justin Pryzby
Reviewed-by: Michaël Paquier
Discussion: https://postgr.es/m/20191012004446.GT10470@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d21f919eb86cd3baa267844d111c6a5af480696

Modified Files
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src/backend/commands/indexcmds.c |  6 ++++--
src/backend/storage/lmgr/lmgr.c  | 10 ++++------
2 files changed, 8 insertions(+), 8 deletions(-)