Fix MSVC warnings about possible value truncation.

src/hdmx.cc

Index: src/hdmx.cc
diff --git a/src/hdmx.cc b/src/hdmx.cc
index c30ac4893fb6b3c9ebf9a526d61e247121138310..570fab1682b2d38cb8623240f3b4beeee273bd5d 100644
--- a/src/hdmx.cc
+++ b/src/hdmx.cc
@@ -105,13 +105,16 @@ bool ots_hdmx_should_serialise(OpenTypeFile *file) {
 bool ots_hdmx_serialise(OTSStream *out, OpenTypeFile *file) {
   OpenTypeHDMX * const hdmx = file->hdmx;
 
-  if (!out->WriteU16(hdmx->version) ||
-      !out->WriteS16(hdmx->records.size()) ||
+  const int16_t num_recs = static_cast<int16_t>(hdmx->records.size());
+  if (hdmx->records.size() >
+          static_cast<size_t>(std::numeric_limits<int16_t>::max()) ||
+      !out->WriteU16(hdmx->version) ||
+      !out->WriteS16(num_recs) ||
       !out->WriteS32(hdmx->size_device_record)) {
     return OTS_FAILURE();
   }
 
-  for (unsigned i = 0; i < hdmx->records.size(); ++i) {
+  for (int16_t i = 0; i < num_recs; ++i) {
     const OpenTypeHDMXDeviceRecord& rec = hdmx->records[i];
     if (!out->Write(&rec.pixel_size, 1) ||
         !out->Write(&rec.max_width, 1) ||