Do not reserve a response body by its claimed length

The client reserved what Content-Length claimed, up to
payload_max_length, before any of the body arrived, and without a limit
nothing bounded it. Reserve at most 1 MiB up front and let a longer body
grow as it is read.

httplib.h

@@ -16599,13 +16599,13 @@ inline bool ClientImpl::process_request(Stream &strm, Request &req,
           output_error_log(error, &req);
           return false;
         }
-        // Cap the reservation by payload_max_length_ to avoid OOM when a
-        // hostile or malformed server sends an enormous Content-Length.
-        // The actual body read below is bounded by payload_max_length_,
-        // so reserving more than that is never useful.
-        auto reserve_len = static_cast<size_t>(len);
-        if (payload_max_length_ > 0 && reserve_len > payload_max_length_) {
-          reserve_len = payload_max_length_;
+        // Content-Length is only the server's claim, so reserve no more than
+        // payload_max_length_ allows, nor more than 1 MiB before any of the
+        // body has arrived. A longer body grows as it is read.
+        auto reserve_len =
+            (std::min)(static_cast<size_t>(len), size_t(1024 * 1024));
+        if (payload_max_length_ > 0) {
+          reserve_len = (std::min)(reserve_len, payload_max_length_);
         }
         res.body.reserve(reserve_len);
       }