refactor: Cast type explicitly

This change:
 - Is a pure refactor without altering the assembly code, which
   might be verified, for example, with the https://godbolt.org/
   service.
 - Fixes warning C4244 when compiling with MSVC at production quality
   W3 warning level.

util/coding.cc

@@ -56,7 +56,7 @@ char* EncodeVarint64(char* dst, uint64_t v) {
   static const int B = 128;
   uint8_t* ptr = reinterpret_cast<uint8_t*>(dst);
   while (v >= B) {
-    *(ptr++) = v | B;
+    *(ptr++) = static_cast<uint8_t>(v | B);
     v >>= 7;
   }
   *(ptr++) = static_cast<uint8_t>(v);