Fix allowing to make null EGL surfaces current.

Fix allowing to make null EGL surfaces current.

eglMakeCurrent(display, EGL_NO_SURFACE, EGL_NO_SURFACE, context)
resulted in a null dereference. It should be supported and is a common
idiom for detaching the surfaces from the context before destroying
them.

Bug swiftshader:70

Change-Id: I5b4406c8d594bc5db34c51bd08371ce69bbd471b
Reviewed-on: https://swiftshader-review.googlesource.com/10389
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>