Check for NULL texture in GrAutoScratchTexture::detach()

R=sugoi@chromium.org, robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://chromiumcodereview.appspot.com/14683009

git-svn-id: http://skia.googlecode.com/svn/trunk@9104 2bbb7eff-a529-9590-31e7-b0007b416f81

include/gpu/GrContext.h

@@ -978,6 +978,9 @@ public:
      * returned texture.
      */
     GrTexture* detach() {
+        if (NULL == fTexture) {
+            return NULL;
+        }
         GrTexture* texture = fTexture;
         fTexture = NULL;