egl/wayland: Don't segfault if `create_wl_buffer` returns `NULL`

egl/wayland: Don't segfault if `create_wl_buffer` returns `NULL`

Normally, this shouldn't fail, but it has various cases where it returns
`NULL`. Without this change, it would result in a segfault when
`wl_buffer_add_listener` is called.

This instead makes `EGLSwapBuffers` return a `EGL_BAD_ALLOC` error.

The other place `create_wl_buffer` is called already checks the return
value, and the Vulkan WSI code doesn't seem to have an issue like this.

Signed-off-by: Ian Douglas Scott <ian@iandouglasscott.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24915>