start and end BN_CTX so BN_CTX can track func owner of temporary BNs
put BN_CTX_end in a finally to ensure it is invoked
extraneous newline murdered
switch to a BN_CTX context manager.
try/finally the bn_ctx manager
Merge pull request #1344 from reaperhulk/bn-ctx-fix

start and end BN_CTX so BN_CTX can track func owner of temporary BNs