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 ab0dfc96, so backpatch to pg12.

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

src/backend/commands/indexcmds.c

@@ -384,10 +384,12 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)
 
 		if (VirtualTransactionIdIsValid(old_snapshots[i]))
 		{
+			/* If requested, publish who we're going to wait for. */
 			if (progress)
 			{
 				PGPROC	   *holder = BackendIdGetProc(old_snapshots[i].backendId);
 
+				if (holder)
 					pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
 												 holder->pid);
 			}

src/backend/storage/lmgr/lmgr.c

@@ -900,14 +900,12 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 
 		while (VirtualTransactionIdIsValid(*lockholders))
 		{
-			/*
-			 * If requested, publish who we're going to wait for.  This is not
-			 * 100% accurate if they're already gone, but we don't care.
-			 */
+			/* If requested, publish who we're going to wait for. */
 			if (progress)
 			{
 				PGPROC	   *holder = BackendIdGetProc(lockholders->backendId);
 
+				if (holder)
 					pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
 												 holder->pid);
 			}