Fix setting next multixid's offset at offset wraparound

In commit 789d653, we started updating the next multixid's offset too when recording a multixid, so that it can always be used to calculate the number of members. I got it wrong at offset wraparound: we need to skip over offset 0. Fix that. Discussion: https://www.postgresql.org/message-id/d9996478-389a-4340-8735-bfad456b313c@iki.fi Backpatch-through: 14

Author: hlinnaka

src/backend/access/transam/multixact.c

@@ -909,6 +909,7 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
 int64next_pageno;
 intnext_entryno;
 MultiXactOffset *next_offptr;
+MultiXactOffset next_offset;
 LWLock *lock;
 LWLock *prevlock = NULL;
 
@@ -976,11 +977,15 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
 next_offptr += next_entryno;
 }
 
-if (*next_offptr != offset + nmembers)
+/* Like in GetNewMultiXactId(), skip over offset 0 */
+next_offset = offset + nmembers;
+if (next_offset == 0)
+next_offset = 1;
+if (*next_offptr != next_offset)
 {
 /* should already be set to the correct value, or not at all */
 Assert(*next_offptr == 0);
-*next_offptr = offset + nmembers;
+*next_offptr = next_offset;
 MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
 }