xvba_video_glx.c: split check in transfer_surface.

The boolean operator || requires an evaluation of both expressions.
If the expressions evaluates pointers, then care must be taken that
both expressions have valid pointer. In this case it cannot be guaranteed,
thus splitting the check is needed.

src/xvba_video_glx.c

@@ -860,7 +860,9 @@ transfer_surface_native(
     GLVTable * const gl_vtable = gl_get_vtable();
 
     object_context_p obj_context = XVBA_CONTEXT(obj_surface->va_context);
-    if (!obj_context || !obj_context->xvba_session)
+    if (!obj_context)
+        return VA_STATUS_ERROR_INVALID_CONTEXT;
+    if (!obj_context->xvba_session)
         return VA_STATUS_ERROR_INVALID_CONTEXT;
 
     /* Create XvBA/GLX surface */