vboxDomainScreenshot: Don't pass uninitialized 'screenData' to VIR_FREE

If one of the early checks to get screen resolution fails 'screenData'
would be passed to VIR_FREE uninitialized. Unfortunately the compiler
isn't able to detect this when VIR_FREE is implemented using
g_clear_pointer.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com

src/vbox/vbox_common.c

@@ -7418,7 +7418,7 @@ vboxDomainScreenshot(virDomainPtr dom,
             if (display) {
                 PRUint32 width, height, bitsPerPixel;
                 PRUint32 screenDataSize;
-                PRUint8 *screenData;
+                PRUint8 *screenData = NULL;
                 PRInt32 xOrigin, yOrigin;
 
                 rc = gVBoxAPI.UIDisplay.GetScreenResolution(display, screen,