Fix #296: Explicitly cast size_t results to unsigned when needed

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

src/lib_json/json_value.cpp

@@ -126,7 +126,7 @@ 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);