Fix second memory leak in SkSpecialImage

TBR=bsalomon@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1825073003

Review URL: https://codereview.chromium.org/1825073003

src/core/SkSpecialImage.cpp

@@ -502,7 +502,8 @@ public:
         desc.fWidth = subset.width();
         desc.fHeight = subset.height();
 
-        GrTexture* subTx = ctx->textureProvider()->createTexture(desc, SkBudgeted::kYes);
+        SkAutoTUnref<GrTexture> subTx(ctx->textureProvider()->createTexture(desc,
+                                                                            SkBudgeted::kYes));
         if (!subTx) {
             return nullptr;
         }