Update error code for cert parsing failure

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index eb3dcc2..97f9c1b 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1898,7 +1898,7 @@
             mbedtls_ssl_send_alert_message( ssl,
                               MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                               MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
-            return( MBEDTLS_ERR_SSL_BAD_CERTIFICATE );
+            return( MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         }
         /* In theory, the CRT can be up to 2**24 Bytes, but we don't support
          * anything beyond 2**16 ~ 64K. */