Make GrGLGetDriverInfo work for null strings.

src/gpu/gl/GrGLUtil.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGLUtil.h"
 #include "SkMatrix.h"
@@ skipping 84 matching lines @@
 void GrGLGetDriverInfo(GrGLStandard standard,
                        GrGLVendor vendor,
                        const char* rendererString,
                        const char* versionString,
                        GrGLDriver* outDriver,
                        GrGLDriverVersion* outVersion) {
     int major, minor, rev, driverMajor, driverMinor;
 
     *outDriver = kUnknown_GrGLDriver;
     *outVersion = GR_GL_DRIVER_UNKNOWN_VER;
+    // These null checks are for test GL contexts that return nullptr in their
+    // glGetString implementation.
+    if (!rendererString) {
+        rendererString = "";
+    }
+    if (!versionString) {
+        versionString = "";
+    }
 
     if (0 == strcmp(rendererString, "Chromium")) {
         *outDriver = kChromium_GrGLDriver;
         return;
     }
 
     if (standard == kGL_GrGLStandard) {
         if (kNVIDIA_GrGLVendor == vendor) {
             *outDriver = kNVIDIA_GrGLDriver;
             int n = sscanf(versionString, "%d.%d.%d NVIDIA %d.%d",
@@ skipping 226 matching lines @@
     GR_STATIC_ASSERT(2 == kGreater_StencilFunc);
     GR_STATIC_ASSERT(3 == kGEqual_StencilFunc);
     GR_STATIC_ASSERT(4 == kLess_StencilFunc);
     GR_STATIC_ASSERT(5 == kLEqual_StencilFunc);
     GR_STATIC_ASSERT(6 == kEqual_StencilFunc);
     GR_STATIC_ASSERT(7 == kNotEqual_StencilFunc);
     SkASSERT((unsigned) basicFunc < kBasicStencilFuncCount);
 
     return gTable[basicFunc];
 }