Fix GLHelperReadbackSupport::SupportsFormat crash

content/common/gpu/client/gl_helper_readback_support.cc

Index: content/common/gpu/client/gl_helper_readback_support.cc
diff --git a/content/common/gpu/client/gl_helper_readback_support.cc b/content/common/gpu/client/gl_helper_readback_support.cc
index 9c1a4ce8ca22767e08cfff1eab2ff7451490be26..e773ca90782a6dc684bec38c7afb455e1cfe925c 100644
--- a/content/common/gpu/client/gl_helper_readback_support.cc
+++ b/content/common/gpu/client/gl_helper_readback_support.cc
@@ -106,10 +106,12 @@ bool GLHelperReadbackSupport::SupportsFormat(GLenum format, GLenum type) {
 
   if (format == GL_BGRA_EXT && type == GL_UNSIGNED_BYTE) {
     const GLubyte* tmp = gl_->GetString(GL_EXTENSIONS);
-    std::string extensions =
-        " " + std::string(reinterpret_cast<const char*>(tmp)) + " ";
-    if (extensions.find(" GL_EXT_read_format_bgra ") != std::string::npos) {
-      return true;
+    if (tmp) {
+      std::string extensions =
+          " " + std::string(reinterpret_cast<const char*>(tmp)) + " ";
+      if (extensions.find(" GL_EXT_read_format_bgra ") != std::string::npos) {
+        return true;
+      }
     }
   }