programmable renderer crashes if currently bound shader is deleted

This is probably an edge case, but I'm finding that if a shader is bound (begin) and then deleted without unbinding (end) in ofApp::draw() while using the programmable renderer, there is a crash on anything that uses the internal currentShader pointer.

I'm running into this while using ofxLua where the script side of the app creates the shader and binds it. A script error then occurs before the shader is unbound and the script state is cleared (including the shader). Perhaps the shader should be unbound before it is cleared as a precaution?

EDIT: This is using the current OF stable release on macOS 10.12.

Start at ofApp::draw() and trace the programmable renderer's currentShader handling when a bound shader is deleted, using the ofxLua failure scenario as a reproduction guide. Verify that deleting a currently bound shader no longer crashes later uses of currentShader and that the shader lifecycle remains safe after script state is cleared.