Add check for I/O log file handle being NULL. This could only

happen if the front-end calls iolog_open with argc == 0 but actually
runs a command.

plugins/sudoers/iolog.c

@@ -719,6 +719,12 @@ sudoers_io_log(const char *buf, unsigned int len, int idx)
     int rval = true;
     debug_decl(sudoers_io_version, SUDOERS_DEBUG_PLUGIN)
 
+    if (io_log_files[idx].fd.v == NULL) {
+	sudo_warnx(U_("%s: internal error, file index %d not open"),
+	    __func__, idx);
+	debug_return_int(-1);
+    }
+
     gettimeofday(&now, NULL);
 
 #ifdef HAVE_ZLIB_H