I would have thought that for our usecase (we only ever access a copy
of an md ctx in a single thread) we do not need this, but due to strange
error output on CentOS 7.2.1511 libcrypto 1.0.1e, add it to be on the
safe side:

-   Debug          "vauth_sig.valid() failed%00"
-   Debug          "crypto 139880179115568:error:04091068:lib(4):func(145):reason(104):rsa_sign.c:287:%0a%00"

The interesting bit is the %00 at the end of both lines