Initialise |supports_digest|.

We were getting this because of C's defaults, but it's fragile to leave
it like this because someone may add another field at the end in the
future.

Change-Id: I8b2dcbbc7cee8062915d15101f99f5a1aae6ad87
Reviewed-on: https://boringssl-review.googlesource.com/5860
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/rsa/rsa_impl.c

@@ -1152,4 +1152,6 @@ const struct rsa_meth_st RSA_default_method = {
 
   keygen,
   keygen_multiprime,
+
+  NULL /* supports_digest */,
 };