Use explicit cast to ptrdiff_t (#267)

In RemoveLast, explicitly cast to ptrdiff_t, rather than relying on implicit conversions when skip is called.

src/s2/util/coding/coder.h

@@ -134,7 +134,7 @@ class Encoder {
 
   // REQUIRES: length() >= N
   // Removes the last N bytes out of the encoded buffer
-  void RemoveLast(size_t N) { writer().skip(-N); }
+  void RemoveLast(size_t N) { writer().skip(-static_cast<ptrdiff_t>(N)); }
 
   // REQUIRES: length() >= N
   // Removes the last length()-N bytes to make the encoded buffer have length N