Add SSL_get_rc4_state.

This allows the current RC4 state of an SSL* to be extracted. We have
internal uses for this functionality.

Change-Id: Ic124c4b253c8325751f49e7a4c021768620ea4b7
Reviewed-on: https://boringssl-review.googlesource.com/3722
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 46b9cb6..04f6c54 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -3140,3 +3140,13 @@
 const SSL_CIPHER *SSL_get_cipher_by_value(uint16_t value) {
   return ssl3_get_cipher_by_value(value);
 }
+
+int SSL_get_rc4_state(const SSL *ssl, const RC4_KEY **read_key,
+                      const RC4_KEY **write_key) {
+  if (ssl->aead_read_ctx == NULL || ssl->aead_write_ctx == NULL) {
+    return 0;
+  }
+
+  return EVP_AEAD_CTX_get_rc4_state(&ssl->aead_read_ctx->ctx, read_key) &&
+         EVP_AEAD_CTX_get_rc4_state(&ssl->aead_write_ctx->ctx, write_key);
+}