v3.2/glfw: Fix Window.SetIcon handling of empty slice. (#194)
v3.2/glfw: Fix Window.SetIcon handling of empty slice. (#194)

Window.SetIcon method is documented as:

	If no images are specified, the window reverts to its default icon.

Fix an index out of range panic when no images are provided.
Use a null pointer in C.glfwSetWindowIcon call instead.

Fixes #193.
go-gl/glfw,go-gl/glfw,gonutz/glfw,go-gl/glfw,gonutz/glfw