Workaround for BN_bn2mpi reading/writing out of bounds

When OpenSSL's BN_bn2mpi is passed a buffer of size 4, valgrind
reports reading/writing one byte past it. I am unable to find
evidence of this behaviour in BN_bn2mpi's source code, so it may
be a spurious warning. However, this change is harmless, as only
the bignum with value 0 results in an mpi serialization of size 4.