Fix CRYPTO_clear_realloc() bug.

If allocation in CRYPTO_clear_realloc() fails don't free up the original
buffer: this is consistent with the behaviour of realloc(3) and is expected
in other places in OpenSSL.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>