Fix panic on shutdown

[davidbyttow]

Upon termination, if libvips continues work in another thread (such as for reporting leaks) and logs, then a panic will occur if the log handler is a callback into Go. This is not an ideal fix because all processes/threads should terminate appropriately, but it's a stopgap to prevent the failure.

There may be a better way to handle this (via a signal callback or something else) to catch it just as Go is exiting and unset the handler.

Users are suggested to call Shutdown() upon exit to avoid this error.

[coveralls]

Coverage decreased (-0.3%) to 81.181% when pulling cd7a815 on fix-#144 into 07ff0af on master.