RAII bssl_shim.

bssl_shim rather needs it. It doesn't even free the SSL* properly most of the
time. Now that it does, this opens the door to running malloc tests under
a leak checker (because it's just not slow enough right now).

Change-Id: I37d2004de27180c41b42a6d9e5aea02caf9b8b32
Reviewed-on: https://boringssl-review.googlesource.com/3340
Reviewed-by: Adam Langley <agl@google.com>