Assert lack of hazardous buffer locks before possible catalog read.

Commit 0bada39c83 fixed a bug of this kind,
which existed in all branches for six days before detection.  While the
probability of reaching the trouble was low, the disruption was extreme.  No
new backends could start, and service restoration needed an immediate
shutdown.  Hence, add this to catch the next bug like it.

The new check in RelationIdGetRelation() suffices to make autovacuum detect
the bug in commit 243e9b40f1 that led to commit
0bada39.  This also checks in a number of similar places.  It replaces each
Assert(IsTransactionState()) that pertained to a conditional catalog read.

Back-patch to v14 - v17.  This a back-patch of commit
f4ece891fc (from before v18 branched) to
all supported branches, to accompany the back-patch of commits 243e9b4
and 0bada39.  For catalog indexes, the bttextcmp() behavior that
motivated IsCatalogTextUniqueIndexOid() was v18-specific.  Hence, this
back-patch doesn't need that or its correction from commit
4a4ee0c2c1.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/20250410191830.0e.nmisch@google.com
Discussion: https://postgr.es/m/10ec0bc3-5933-1189-6bb8-5dec4114558e@gmail.com
Backpatch-through: 14-17

src/backend/utils/mb/mbutils.c

@@ -39,6 +39,7 @@
 #include "mb/pg_wchar.h"
 #include "utils/fmgrprotos.h"
 #include "utils/memutils.h"
+#include "utils/relcache.h"
 #include "varatt.h"
 
 /*
@@ -310,7 +311,7 @@ InitializeClientEncoding(void)
 	{
 		Oid			utf8_to_server_proc;
 
-		Assert(IsTransactionState());
+		AssertCouldGetRelation();
 		utf8_to_server_proc =
 			FindDefaultConversionProc(PG_UTF8,
 									  current_server_encoding);