Stbt record failsafe with no frame buffer

This branch contains 2 commits which aim to overcome a problem seen with stbt-record
when frames are not available on the frame-buffer (for whatever reason), cause stbt-record
to fatal error when it attempts to save the current frame to disk.

The first commit (which modifies stbt-record.Display class) is not necessarily complete per se; it does what is needed but I think the code could possibly be optimised. I don't know if the way I am restarting the pipeline is correct.