fix github issue #832 - "nethack --nethackrc=file"

Issue reported by schauveau:  when the command line specifies the
'--nethackrc=file' option (Unix, including linux and OSX, only) the
value of 'file' was set in pointer g.cmdline_rcfile and the options
parsing made a copy of that pointer rather than of what it pointed
to, then freed g.cmdline_rcfile before using the copy.  So it ended
up using a pointer to freed memory.

Stick with copying the pointer but defer freeing the original until
after its copy is no longer needed.

Fixes #832

doc/fixes3-7-0.txt

@@ -1331,6 +1331,9 @@ when one monster swallowed/engulfed another and killed it, impossible
 	"placing <mon> over itself ..." could be given by place_monster()
 if the game crashed or the process was killed, recovering a save file ended
 	up with incorrect stairway connections between levels
+when using --nethackrc=file on the command line (currently only implemented
+	for ports that use unixmain.c), options parsing freed the string
+	containing 'file' before using it as the RC file name
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support

src/options.c

@@ -6233,7 +6233,6 @@ initoptions_finish(void)
     if (g.cmdline_rcfile) {
         namesrc = "command line";
         nameval = g.cmdline_rcfile;
-        free((genericptr_t) g.cmdline_rcfile), g.cmdline_rcfile = 0;
         xtraopts = opts;
         if (opts && (*opts == '/' || *opts == '\\' || *opts == '@'))
             xtraopts = 0; /* NETHACKOPTIONS is a file name; ignore it */
@@ -6274,6 +6273,9 @@ initoptions_finish(void)
         (void) parseoptions(xtraopts, TRUE, FALSE);
         config_error_done();
     }
+
+    if (g.cmdline_rcfile)
+        free((genericptr_t) g.cmdline_rcfile), g.cmdline_rcfile = 0;
     /*[end of nethackrc handling]*/
 
     (void) fruitadd(g.pl_fruit, (struct fruit *) 0);