MINOR: mworker: set the master side of ipc_fd in the worker to -1

Once the child->ipc_fd[0] is closed in the worker, set the value to -1
so we don't reference a closed FD anymore.
diff --git a/src/haproxy.c b/src/haproxy.c
index 70ce708..eb6c520 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -3302,8 +3302,10 @@
 				 * workers, we don't need to close the worker
 				 * side of other workers since it's done with
 				 * the bind_proc */
-				if (child->ipc_fd[0] >= 0)
+				if (child->ipc_fd[0] >= 0) {
 					close(child->ipc_fd[0]);
+					child->ipc_fd[0] = -1;
+				}
 				if (child->options & PROC_O_TYPE_WORKER &&
 				    child->reloads == 0) {
 					/* keep this struct if this is our pid */