made a bit more robust

modules/src/system/close.c

@@ -9,7 +9,9 @@
 sys_close(fp)
 	register File *fp;
 {
-	fp->o_flags = 0;
-	close(fp->o_fd);
-	fp->o_fd = -1;
+	if (fp) {
+		fp->o_flags = 0;
+		close(fp->o_fd);
+		fp->o_fd = -1;
+	}
 }

modules/src/system/read.c

@@ -12,5 +12,6 @@ sys_read(fp, bufptr, bufsiz, pnbytes)
 	char *bufptr;
 	int bufsiz, *pnbytes;
 {
+	if (! fp) return 0;
 	return (*pnbytes = read(fp->o_fd, bufptr, bufsiz)) >= 0;
 }

modules/src/system/seek.c

@@ -14,5 +14,6 @@ sys_seek(fp, off, whence, poff)
 	long off;
 	long *poff;
 {
+	if (! fp) return 0;
 	return (*poff = lseek(fp->o_fd, off, whence)) >= 0;
 }

modules/src/system/write.c

@@ -12,5 +12,6 @@ sys_write(fp, bufptr, nbytes)
 	char *bufptr;
 	int nbytes;
 {
+	if (! fp) return 0;
 	return write(fp->o_fd, bufptr, nbytes) == nbytes;
 }