Add tests (and fix!) for known bad ICO files.

src/images/SkImageDecoder_libico.cpp

Index: src/images/SkImageDecoder_libico.cpp
diff --git a/src/images/SkImageDecoder_libico.cpp b/src/images/SkImageDecoder_libico.cpp
index cd8a292edc149d70f8cf46f6657e5df5052c0691..5240d09b8672dbdd1980b27bf7d959b8dbcb425f 100644
--- a/src/images/SkImageDecoder_libico.cpp
+++ b/src/images/SkImageDecoder_libico.cpp
@@ -159,7 +159,7 @@ SkImageDecoder::Result SkICOImageDecoder::onDecode(SkStream* stream, SkBitmap* b
     const size_t size = read4Bytes(buf, 14 + choice*16);           //matters?
     const size_t offset = read4Bytes(buf, 18 + choice*16);
     // promote the sum to 64-bits to avoid overflow
-    if (((uint64_t)offset + size) > length) {
+    if (offset > length || size > length || ((uint64_t)offset + size) > length) {
         return kFailure;
     }