preproc.c: Don't forget to dup filename before free

src_set_fname simply gets copy of pointer (ideally
we need refcounting here) so don't pass the name
which will be freed soon but rather pass a copy.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

preproc.c

@@ -5006,7 +5006,7 @@ static char *pp_getline(void)
                 /* only set line and file name if there's a next node */
                 if (i->next) {
                     src_set_linnum(i->lineno);
-                    nasm_free(src_set_fname(i->fname));
+                    nasm_free(src_set_fname(nasm_strdup(i->fname)));
                 }
                 istk = i->next;
                 list->downlevel(LIST_INCLUDE);