Ensure chart.ctx exists during deferred updates

[BR0kEN-]

Ensure the chart instance is still alive. It may have been destroyed during a delay and calling chart.update() will fail. The most common reason for such scenario is user navigation.

Steps:

• open a view with the chart that has a deferred render;
• close a view before the delay timer expires.

After the second step both chart.ctx and chart.canvas won't exist.

Note that chart.canvas is a reference of chart.ctx.canvas.

Fixes #13, #16

[simonbrunel]

Merged in ed96880df3a93490c05fda44942334ce0bdabf83.

Thanks a lot @BR0kEN-!