Make sure the cursor’s buffer is valid before destroying it

[linkmauve]

The shape should never be an unknown value, but it is imo still better to check it really is. Another way would be to abort() in translateCursorShape(), which I find less elegant than this one.

The other change is there to handle the case where the cursor creation failed, so we don’t buffer_destroy NULL.

[elmindreda]

The test in glfwPlatformDestroyCursor is good, but glfwCreateStandardCursor already verifies that the shape value is valid.

[linkmauve]

I removed the first test, and changed the commit message.

[elmindreda]

Thank you!