skimage: if --readPath points at an empty dir, create an empty expectations file

TBR=scroggo

Review URL: https://codereview.chromium.org/17294006

git-svn-id: http://skia.googlecode.com/svn/trunk@9642 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index 107d3bf..b115392 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -515,7 +515,8 @@
                 decodeFileAndWrite(fullname.c_str(), outDirPtr);
             } while (iter.next(&filename));
         } else {
-            decodeFileAndWrite(FLAGS_readPath[i], outDirPtr);
+            // FLAGS_readPath[i] is an empty or nonexistent directory,
+            // so do nothing.
         }
     }