Release memory in case of error in the OpenSSL ECDSA constructor

Release memory in case of error in the OpenSSL ECDSA constructor

The current code will only fail to release the allocated memory if
called with an invalid algorithm, which won't happen, or if a
memory allocation fails in which case this might not matter much.
Still, it's cleaner to release the memory properly and might avoid
mistakes later if we look at this code while implementing a new
crypto backend.