minor code fix for -g option on MSDOS

[Paul Eggert]

I installed this patch, found by code inspection:

2002-12-25  Paul Eggert  <address@hidden>

        * src/getargs.c (getargs) [MSDOS]: Don't assume optarg != NULL
        with -g option.

--- getargs.c   13 Dec 2002 11:41:21 -0000      1.53
+++ getargs.c   26 Dec 2002 06:55:45 -0000      1.54
@@ -355,7 +355,8 @@ getargs (int argc, char *argv[])
       case 'g':
        /* Here, the -g and --graph=FILE options are differentiated.  */
        graph_flag = 1;
-       spec_graph_file = AS_FILE_NAME (optarg);
+       if (optarg)
+         spec_graph_file = AS_FILE_NAME (optarg);
        break;
 
       case 'v':