socketshared: add maximum message size to avoid very large allocations (#40050)

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>

Author: benhillis

src/shared/inc/socketshared.h

@@ -64,6 +64,15 @@ try
 #endif
     }
 
+    if (MessageSize > 4 * 1024 * 1024) // 4 MiB
+    {
+#if defined(_MSC_VER)
+        THROW_HR_MSG(E_UNEXPECTED, "Message size too large: %llu", MessageSize);
+#elif defined(__GNUC__)
+        THROW_UNEXCEPTED();
+#endif
+    }
+
     if (Buffer.size() < MessageSize)
     {
         Buffer.resize(MessageSize);