cdvdgigaherz: Use null for event/thread handle init

CreateEvent and CreateThread return NULL on failure, not
INVALID_HANDLE_VALUE. This should have been done in
0477e03965, I didn't check thoroughly
enough.

plugins/cdvdGigaherz/src/CDVD.cpp

@@ -25,8 +25,8 @@
 
 void (*newDiscCB)();
 
-HANDLE hThread_keepAlive = INVALID_HANDLE_VALUE;
-HANDLE hNotify_keepAlive = INVALID_HANDLE_VALUE;
+HANDLE hThread_keepAlive = nullptr;
+HANDLE hNotify_keepAlive = nullptr;
 DWORD  pidThreadKeepAlive = 0;
 
 #define STRFY(x) #x

plugins/cdvdGigaherz/src/ReadThread.cpp

@@ -21,9 +21,9 @@ s32 prefetch_last_lba=0;
 s32 prefetch_last_mode=0;
 s32 prefetch_left=0;
 
-HANDLE hNotify = INVALID_HANDLE_VALUE;
-HANDLE hThread = INVALID_HANDLE_VALUE;
-HANDLE hRequestComplete = INVALID_HANDLE_VALUE;
+HANDLE hNotify = nullptr;
+HANDLE hThread = nullptr;
+HANDLE hRequestComplete = nullptr;
 
 CRITICAL_SECTION CacheMutex;
 

plugins/cdvdGigaherz/src/Windows/IOCtlSrc.cpp

@@ -295,6 +295,7 @@ s32 IOCtlSrc::Reopen()
 	// Dual layer DVDs cannot read from layer 1 without this ioctl
 	DeviceIoControl(device, FSCTL_ALLOW_EXTENDED_DASD_IO, nullptr, 0, nullptr, 0, &size, nullptr);
 
+	// FIXME: 0 is a valid session id, but the code assumes that it isn't.
 	sessID=0;
 	DeviceIoControl(device,IOCTL_DVD_START_SESSION,NULL,0,&sessID,sizeof(DVD_SESSION_ID), &size, NULL);