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/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;