Fix overflow when comparing two ints by promoting the sum to 64-bits.

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 90058d679af3ae2450d827a2bc8e212ab5183b4c..f75d80439f5c3cc2b07baed38fff90d79acfda4f 100644
--- a/src/images/SkImageDecoder_libico.cpp
+++ b/src/images/SkImageDecoder_libico.cpp
@@ -154,7 +154,8 @@ bool SkICOImageDecoder::onDecode(SkStream* stream, SkBitmap* bm, Mode mode)
     //int fakeBitCount = read2Bytes(buf, 12 + choice*16); //should be real - usually 0
     const size_t size = read4Bytes(buf, 14 + choice*16);           //matters?
     const size_t offset = read4Bytes(buf, 18 + choice*16);
-    if ((offset + size) > length) {
+    // promote the sum to 64-bits to avoid overflow
+    if (((uint64_t)offset + size) > length) {
         return false;
     }