In the error cleanup path in wlEglCreatePlatformWindowSurfaceHook, don't
try to dereference the WlEglSurface if we never allocated it.

In wlEglCreatePlatformWindowSurfaceHook, check if there's already a
EGLSurface that uses the same wl_surface object, and if so, fail with
EGL_BAD_ALLOC.

We've got a check (using the wl_egl_window::driver_private pointer) to
catch if the app tries to create multiple EGLSurfaces from the same
wl_egl_window. But, an app could still call wl_egl_window_create
multiple times, which would give it multiple wl_egl_window structs for
the same wl_surface.