Silence assorted "variable may be used uninitialized" warnings.

All of these are false positives, but in each case a fair amount of
analysis is needed to see that, and it's not too surprising that not all
compilers are smart enough.  (In particular, in the logtape.c case, a
compiler lacking the knowledge provided by the Assert would almost surely
complain, so that this warning will be seen in any non-assert build.)

Some of these are of long standing while others are pretty recent,
but it only seems worth fixing them in HEAD.

Jaime Casanova, tweaked a bit by me

Discussion: https://postgr.es/m/CAJGNTeMcYAMJdPAom52dppLMtF-UnEZi0dooj==75OEv1EoBZA@mail.gmail.com

src/backend/access/transam/xloginsert.c

@@ -584,7 +584,7 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 		if (include_image)
 		{
 			Page		page = regbuf->page;
-			uint16		compressed_len;
+			uint16		compressed_len = 0;
 
 			/*
 			 * The page needs to be backed up, so calculate its hole length

src/backend/catalog/objectaddress.c

@@ -1593,6 +1593,8 @@ get_object_address_opf_member(ObjectType objtype,
 	famaddr = get_object_address_opcf(OBJECT_OPFAMILY, copy, false);
 
 	/* find out left/right type names and OIDs */
+	typenames[0] = typenames[1] = NULL;
+	typeoids[0] = typeoids[1] = InvalidOid;
 	i = 0;
 	foreach(cell, lsecond(object))
 	{

src/backend/utils/sort/logtape.c

@@ -411,7 +411,7 @@ ltsConcatWorkerTapes(LogicalTapeSet *lts, TapeShare *shared,
 					 SharedFileSet *fileset)
 {
 	LogicalTape *lt = NULL;
-	long		tapeblocks;
+	long		tapeblocks = 0L;
 	long		nphysicalblocks = 0L;
 	int			i;
 

src/bin/pgbench/pgbench.c

@@ -1495,6 +1495,7 @@ coerceToBool(PgBenchValue *pval, bool *bval)
 	else /* NULL, INT or DOUBLE */
 	{
 		fprintf(stderr, "cannot coerce %s to boolean\n", valueTypeName(pval));
+		*bval = false;			/* suppress uninitialized-variable warnings */
 		return false;
 	}
 }
@@ -1725,9 +1726,9 @@ evalLazyFunc(TState *thread, CState *st,
  * which do not require lazy evaluation.
  */
 static bool
-evalStandardFunc(
-	TState *thread, CState *st,
-	PgBenchFunction func, PgBenchExprLink *args, PgBenchValue *retval)
+evalStandardFunc(TState *thread, CState *st,
+				 PgBenchFunction func, PgBenchExprLink *args,
+				 PgBenchValue *retval)
 {
 	/* evaluate all function arguments */
 	int				nargs = 0;