Add some sanity checks for the fatal error condition

Sometimes at the top level of the state machine code we know we are
supposed to be in a fatal error condition. This commit adds some sanity
checks to ensure that SSLfatal() has been called.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4778)

ssl/ssl_err.c

diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index 88e741e..b9bef5a 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -877,6 +877,7 @@
     "missing dsa signing cert"},
     {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_MISSING_ECDSA_SIGNING_CERT),
     "missing ecdsa signing cert"},
+    {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_MISSING_FATAL), "missing fatal"},
     {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_MISSING_RSA_CERTIFICATE),
     "missing rsa certificate"},
     {ERR_PACK(ERR_LIB_SSL, 0, SSL_R_MISSING_RSA_ENCRYPTING_CERT),