Silence compiler warnings from some older compilers.

Since a117cebd6, some older gcc versions issue "variable may be used
uninitialized in this function" complaints for brin_summarize_range.
Silence that using the same coding pattern as in bt_index_check_internal;
arguably, a117cebd6 had too narrow a view of which compilers might give
trouble.

Nathan Bossart and Tom Lane.  Back-patch as the previous commit was.

Discussion: https://postgr.es/m/20220601163537.GA2331988@nathanxps13

contrib/amcheck/verify_nbtree.c

@@ -284,7 +284,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 	else
 	{
 		heaprel = NULL;
-		/* for "gcc -Og" https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78394 */
+		/* Set these just to suppress "uninitialized variable" warnings */
 		save_userid = InvalidOid;
 		save_sec_context = -1;
 		save_nestlevel = -1;

src/backend/access/brin/brin.c

@@ -1051,7 +1051,13 @@ brin_summarize_range(PG_FUNCTION_ARGS)
 		save_nestlevel = NewGUCNestLevel();
 	}
 	else
+	{
 		heapRel = NULL;
+		/* Set these just to suppress "uninitialized variable" warnings */
+		save_userid = InvalidOid;
+		save_sec_context = -1;
+		save_nestlevel = -1;
+	}
 
 	indexRel = index_open(indexoid, ShareUpdateExclusiveLock);