fix dir_cancel_watch issue: fail silently if watch=0 (happens if directory mounted only from archive, i.e. there's no real directory to watch)

This was SVN commit r1524.

source/lib/res/vfs.cpp

@@ -419,7 +419,7 @@ struct TDir
 	TDir()
 	{
 #ifndef NO_DIR_WATCH
-		watch = -1;
+		watch = 0;	// not yet initialized
 #endif
 		loc = 0;
 		exact_dir_name = 0;	// safety

source/lib/sysdep/win/wdir_watch.cpp

@@ -247,7 +247,7 @@ fail:
 
 int dir_cancel_watch(const intptr_t reqnum)
 {
-	if(reqnum < 0)
+	if(reqnum <= 0)
 		return ERR_INVALID_PARAM;
 
 	Watch* w = watches[reqnum];