Fix FlatSize integer overflow.

PiperOrigin-RevId: 871033280

ruy/mat.h

diff --git a/ruy/mat.h b/ruy/mat.h
index 520d717..bf02c66 100644
--- a/ruy/mat.h
+++ b/ruy/mat.h
@@ -350,7 +350,7 @@
 }
 
 inline std::ptrdiff_t FlatSize(const PMatLayout& layout) {
-  const int outerdim =
+  const std::ptrdiff_t outerdim =
       layout.order == Order::kColMajor ? layout.cols : layout.rows;
   return layout.stride * outerdim;
 }