Fix validating sync object pointers.

The spec states that "If <sync> is not a valid sync object for <dpy>,
EGL_FALSE is returned and an EGL_BAD_PARAMETER error is generated."

Change-Id: I4bbd2ddfefd7baba9c301decb644bfb545e66f01
Reviewed-on: https://swiftshader-review.googlesource.com/5290
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>