Protect against transfer.abort() being called during callback

This was already handled in most cases, but not for a body-less response. It's
safe to call transfer.abort() during a callback, so long as the performing flag
is set to true. This was set during the normal libcurl callbacks, but for a
body-less response, we manually invoke the header_done_callback and were not
setting the performing flag.

src/testing.zig

@@ -561,6 +561,14 @@ fn testHTTPHandler(req: *std.http.Server.Request) !void {
         });
     }
 
+    if (std.mem.eql(u8, path, "/xhr_empty")) {
+        return req.respond("", .{
+            .extra_headers = &.{
+                .{ .name = "Content-Type", .value = "text/html; charset=utf-8" },
+            },
+        });
+    }
+
     if (std.mem.eql(u8, path, "/xhr/json")) {
         return req.respond("{\"over\":\"9000!!!\",\"updated_at\":1765867200000}", .{
             .extra_headers = &.{