bmcweb: Add nullptr check for weakptr

When we call a null weakptr, it will cause a crash.
Add nullptr check for weakptr can avoid this situation.

Tested:
bmcweb.service did not experience core-dump.

Change-Id: I4490d68c70ea5d43681f4fb18b3859afb01ed70a
Signed-off-by: Zhao Gang <zhaogang.0108@bytedance.com>
diff --git a/http/websocket.hpp b/http/websocket.hpp
index 027ab24..4f5b3c1 100644
--- a/http/websocket.hpp
+++ b/http/websocket.hpp
@@ -162,6 +162,11 @@
                        [weak(weak_from_this()), onDone{std::move(onDone)}](
                            const boost::beast::error_code& ec, size_t) {
             std::shared_ptr<Connection> self = weak.lock();
+            if (!self)
+            {
+                BMCWEB_LOG_ERROR("Connection went away");
+                return;
+            }
 
             // Call the done handler regardless of whether we
             // errored, but before we close things out