fix(PIWaitEvent): use -1 as pipe_fd sentinel instead of 0

File descriptor 0 is stdin (a valid fd). Using 0 as the 'not-open'
sentinel caused isCreate() to return false and destroy() to skip
close() when pipe() happened to allocate fds {0, 1} (e.g. in daemons
with closed stdio). POSIX convention: -1 = invalid fd.

libs/main/core/piwaitevent_p.h

@@ -55,7 +55,7 @@ private:
 #ifdef WINDOWS
 	void * event = nullptr;
 #else
-	int pipe_fd[2] = {0, 0};
+	int pipe_fd[2] = {-1, -1};
 	fd_set fds[3];
 	enum {
 		ReadEnd  = 0,