Fix open-source-parsers#296: Explicitly cast size_t results to unsigned when needed

This is rebased from open-source-parsers#297, where AppVeyor had been failing, and which
was not properly based on the master branch.

Author: fivemicro

src/lib_json/json_value.cpp

@@ -125,7 +125,7 @@ inline static void decodePrefixedString(
     unsigned* length, char const** value)
 {
   if (!isPrefixed) {
-    *length = strlen(prefixed);
+    *length = static_cast<unsigned>(strlen(prefixed));
     *value = prefixed;
   } else {
     *length = *reinterpret_cast<unsigned const*>(prefixed);

src/lib_json/json_writer.cpp

@@ -354,7 +354,7 @@ void FastWriter::writeValue(const Value& value) {
       const std::string& name = *it;
       if (it != members.begin())
         document_ += ',';
-      document_ += valueToQuotedStringN(name.data(), name.length());
+      document_ += valueToQuotedStringN(name.data(), static_cast<unsigned>(name.length()));
       document_ += yamlCompatiblityEnabled_ ? ": " : ":";
       writeValue(value[name]);
     }
@@ -914,7 +914,7 @@ void BuiltStyledStreamWriter::writeValue(Value const& value) {
         std::string const& name = *it;
         Value const& childValue = value[name];
         writeCommentBeforeValue(childValue);
-        writeWithIndent(valueToQuotedStringN(name.data(), name.length()));
+        writeWithIndent(valueToQuotedStringN(name.data(), static_cast<unsigned>(name.length())));
         *sout_ << colonSymbol_;
         writeValue(childValue);
         if (++it == members.end()) {