Fix BitmapCopy_Texture test on iOS

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1573273002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1580163002 .
diff --git a/tests/BitmapCopyTest.cpp b/tests/BitmapCopyTest.cpp
index e47a631..58341dc 100644
--- a/tests/BitmapCopyTest.cpp
+++ b/tests/BitmapCopyTest.cpp
@@ -684,6 +684,10 @@
                 SkAutoTUnref<GrTexture> texture(
                     ctx->textureProvider()->createTexture(desc, false, srcData, 0));
 
+                if (!texture) {
+                    continue;
+                }
+
                 SkBitmap srcBmp;
                 GrWrapTextureInBitmap(texture, 2, 2, false, &srcBmp);
                 if (srcBmp.isNull()) {