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>

crypto/cipher/e_aes.c

diff --git a/crypto/cipher/e_aes.c b/crypto/cipher/e_aes.c
index e431e0b..7744c00 100644
--- a/crypto/cipher/e_aes.c
+++ b/crypto/cipher/e_aes.c
@@ -1076,6 +1076,7 @@
     aead_aes_gcm_cleanup,
     aead_aes_gcm_seal,
     aead_aes_gcm_open,
+    NULL,                     /* get_rc4_state */
 };
 
 static const EVP_AEAD aead_aes_256_gcm = {
@@ -1088,6 +1089,7 @@
     aead_aes_gcm_cleanup,
     aead_aes_gcm_seal,
     aead_aes_gcm_open,
+    NULL,                     /* get_rc4_state */
 };
 
 const EVP_AEAD *EVP_aead_aes_128_gcm(void) { return &aead_aes_128_gcm; }
@@ -1346,6 +1348,7 @@
     aead_aes_key_wrap_cleanup,
     aead_aes_key_wrap_seal,
     aead_aes_key_wrap_open,
+    NULL,  /* get_rc4_state */
 };
 
 static const EVP_AEAD aead_aes_256_key_wrap = {
@@ -1358,6 +1361,7 @@
     aead_aes_key_wrap_cleanup,
     aead_aes_key_wrap_seal,
     aead_aes_key_wrap_open,
+    NULL,  /* get_rc4_state */
 };
 
 const EVP_AEAD *EVP_aead_aes_128_key_wrap(void) { return &aead_aes_128_key_wrap; }