Run PVS-STUDIO analyzer

Fix issues after a run of PVS-STUDIO analyzer.
Mainly false positives but warnings are anyhow
useful to point out not very readable code.

Noteworthy is the memset() one, where PVS prefers ss-2
instead of stack. This is because memeset() could
be optimized away by the compiler when using 'stack',
due to stack being a local variable no more used after
memset. This should normally not happen, but when
it happens it leads to very sublte and difficult
to find bug, so better to be safe than sorry.

No functional change.

src/main.cpp

@@ -47,4 +47,5 @@ int main(int argc, char* argv[]) {
   UCI::loop(argc, argv);
 
   Threads.exit();
+  return 0;
 }