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.cpp

@@ -65,9 +65,9 @@ void PIWaitEvent::destroy() {
 	}
 #else
 	for (int i = 0; i < 2; ++i) {
-		if (pipe_fd[i] != 0) {
+		if (pipe_fd[i] != -1) {
 			::close(pipe_fd[i]);
-			pipe_fd[i] = 0;
+			pipe_fd[i] = -1;
 		}
 	}
 #endif
@@ -140,7 +140,7 @@ bool PIWaitEvent::isCreate() const {
 #ifdef WINDOWS
 	return event;
 #else
-	return pipe_fd[ReadEnd] != 0;
+	return pipe_fd[ReadEnd] != -1;
 #endif
 }