Avoid unreferenced formal parameter warning in get_range_offset_and_length (yhirose#1838)

Release builds result in the following warning because `content_length` param was used only inside asserts:

1> cpp-httplib\httplib.h(4933,45): warning C4100: 'content_length': unreferenced formal parameter

Author: pps83

httplib.h

@@ -4935,7 +4935,7 @@ get_range_offset_and_length(Range r, size_t content_length) {
   assert(0 <= r.first && r.first < static_cast<ssize_t>(content_length));
   assert(r.first <= r.second &&
          r.second < static_cast<ssize_t>(content_length));
-
+  (void)(content_length);
   return std::make_pair(r.first, static_cast<size_t>(r.second - r.first) + 1);
 }