Fix --set-mtime-command.

This fixes a bug introduced by 281e03ec6.

* src/system.c (sys_exec_setmtime_script): Increase buflen by
value of nread.

src/system.c

@@ -993,7 +993,7 @@ sys_exec_setmtime_script (const char *script_name,
 	    }
 	  if (nread == 0)
 	    break;
-	  buflen += n;
+	  buflen += nread;
 	}
       else if (pfd.revents & POLLHUP)
 	break;