Add a stub for SSL_get_shared_ciphers().

This stub returns an empty string rather than NULL (since some callers
might assume that NULL means there are no shared ciphers).

Change-Id: I9537fa0a80c76559b293d8518599b68fd9977dd8
Reviewed-on: https://boringssl-review.googlesource.com/7196
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/ssl.h

@@ -3064,6 +3064,10 @@ OPENSSL_EXPORT int SSL_renegotiate(SSL *ssl);
 /* SSL_set_state does nothing. */
 OPENSSL_EXPORT void SSL_set_state(SSL *ssl, int state);
 
+/* SSL_get_shared_ciphers writes an empty string to |buf| and returns a
+ * pointer to |buf|, or NULL if |len| is less than or equal to zero. */
+OPENSSL_EXPORT char *SSL_get_shared_ciphers(const SSL *ssl, char *buf, int len);
+
 /* SSL_MODE_HANDSHAKE_CUTTHROUGH is the same as SSL_MODE_ENABLE_FALSE_START. */
 #define SSL_MODE_HANDSHAKE_CUTTHROUGH SSL_MODE_ENABLE_FALSE_START
 

ssl/ssl_lib.c

@@ -1981,6 +1981,14 @@ int SSL_state(const SSL *ssl) { return ssl->state; }
 
 void SSL_set_state(SSL *ssl, int state) { }
 
+char *SSL_get_shared_ciphers(const SSL *ssl, char *buf, int len) {
+  if (len <= 0) {
+    return NULL;
+  }
+  buf[0] = '\0';
+  return buf;
+}
+
 void SSL_set_verify_result(SSL *ssl, long result) {
   ssl->verify_result = result;
 }