Color formatting fixed for wide strings (fixes issue #1594) (#1596)

* Use std::char_traits::length for ansi_color_escape::begin

-Fixes issue #1594 https://github.com/fmtlib/fmt/issues/1594

include/fmt/color.h

@@ -412,7 +412,7 @@ template <typename Char> struct ansi_color_escape {
 
   FMT_CONSTEXPR const Char* begin() const FMT_NOEXCEPT { return buffer; }
   FMT_CONSTEXPR const Char* end() const FMT_NOEXCEPT {
-    return buffer + std::strlen(buffer);
+    return buffer + std::char_traits<Char>::length(buffer);
   }
 
  private: