Fix memory leak in Windows networking subsystem (#5096)

The WSAEWOULDBLOCK case in pony_os_writev and pony_os_send returned
without calling iocp_destroy, leaking the iocp_t and permanently
elevating the token's refcount.

Closes #5090

Author: SeanTAllen

.release-notes/fix-iocp-writev-send-leak.md

@@ -0,0 +1,3 @@
+## Fix memory leak in Windows networking subsystem
+
+Fixed a memory leak on Windows where an IOCP token's reference count was not decremented when a network send operation encountered backpressure. Over time, this could cause memory to grow unboundedly in programs with sustained network traffic.

src/libponyrt/lang/socket.c

@@ -986,6 +986,7 @@ PONY_API size_t pony_os_writev(asio_event_t* ev, LPWSABUF wsa, int wsacnt)
       case WSA_IO_PENDING:
         return wsacnt;
       case WSAEWOULDBLOCK :
+        iocp_destroy(iocp);
         return 0;
       default:
         iocp_destroy(iocp);
@@ -1031,6 +1032,7 @@ PONY_API size_t pony_os_send(asio_event_t* ev, const char* buf, size_t len)
       case WSA_IO_PENDING:
         return len;
       case WSAEWOULDBLOCK :
+        iocp_destroy(iocp);
         return 0;
       default:
         iocp_destroy(iocp);