Add missing NGHTTP2_ERR_BAD_PREFACE to nghttp2_strerror

lib/nghttp2_helper.c

@@ -225,6 +225,8 @@ const char* nghttp2_strerror(int error_code)
     return "Out of memory";
   case NGHTTP2_ERR_CALLBACK_FAILURE:
     return "The user callback function failed";
+  case NGHTTP2_ERR_BAD_PREFACE:
+    return "Received bad connection preface";
   default:
     return "Unknown error code";
   }