All the finalizer-enabled C wrappers must be careful to use
runtime.KeepAlive to ensure the C wrapper object (a Go object)
lives through the end of every C call using state that the
wrapper's finalizer would free.
This CL makes the wrappers appropriately careful.
The test proves that this is the bug I was chasing in a
separate real program, and that the KeepAlives fix it.
I did not write a test of every possible operation.
Change-Id: I627007e480f16adf8396e7f796b54e5525d9ea80
Reviewed-on: https://go-review.googlesource.com/64870
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Adam Langley <agl@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>