Fix Jbig2 document context creation by checking proper pointer

BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 1575 matching lines @@
 }
 
 test("pdfium_embeddertests") {
   sources = [
     "core/fpdfapi/fpdf_page/fpdf_page_func_embeddertest.cpp",
     "core/fpdfapi/fpdf_parser/cpdf_parser_embeddertest.cpp",
     "core/fpdfapi/fpdf_parser/cpdf_security_handler_embeddertest.cpp",
     "core/fpdfapi/fpdf_parser/fpdf_parser_decode_embeddertest.cpp",
     "core/fpdfapi/fpdf_render/fpdf_render_loadimage_embeddertest.cpp",
     "core/fpdfapi/fpdf_render/fpdf_render_pattern_embeddertest.cpp",
+    "core/fxcodec/codec/fx_codec_embeddertest.cpp",
     "core/fxge/ge/fx_ge_text_embeddertest.cpp",
     "fpdfsdk/fpdf_dataavail_embeddertest.cpp",
     "fpdfsdk/fpdfdoc_embeddertest.cpp",
     "fpdfsdk/fpdfedit_embeddertest.cpp",
     "fpdfsdk/fpdfext_embeddertest.cpp",
     "fpdfsdk/fpdfformfill_embeddertest.cpp",
     "fpdfsdk/fpdfsave_embeddertest.cpp",
     "fpdfsdk/fpdftext_embeddertest.cpp",
     "fpdfsdk/fpdfview_c_api_test.c",
     "fpdfsdk/fpdfview_c_api_test.h",
@@ skipping 30 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }