vaapioverlay is only registered if the VA driver support the blend
operation.
This patch only executes the test if vaapioverlay is available,
otherwise the test is bail out without raising an error.
This issue was detected by Coverity.
If the function returns an error value, the error value may be mistaken
for a normal value.
In cb_mouse_event: Value returned from a function is not checked for
errors before being used