Fix two more warnings

src/utils/string.cpp

@@ -15,11 +15,11 @@ std::vector<std::string> cut(const std::string& val, const std::size_t size)
       // Get the number of chars, <= size, that contain only whole
       // UTF-8 codepoints.
       std::size_t s = 0;
-      auto codepoint_size = utils::get_next_codepoint_size(val[pos + s]);
+      auto codepoint_size = utils::get_next_codepoint_size(static_cast<unsigned char>(val[pos + s]));
       while (s + codepoint_size <= size && pos + s < val.size())
         {
           s += codepoint_size;
-          codepoint_size = utils::get_next_codepoint_size(val[pos + s]);
+          codepoint_size = utils::get_next_codepoint_size(static_cast<unsigned char>(val[pos + s]));
         }
       res.emplace_back(val.substr(pos, s));
       pos += s;