winebuild: Don't segfault when run without arguments.

tools/winebuild/main.c

@@ -142,13 +142,13 @@ static void set_dll_file_name( const char *name, DLLSPEC *spec )
 /* set the dll name from the file name */
 static void init_dll_name( DLLSPEC *spec )
 {
-    if (!spec->file_name)
+    if (!spec->file_name && output_file_name)
     {
         char *p;
         spec->file_name = xstrdup( output_file_name );
         if ((p = strrchr( spec->file_name, '.' ))) *p = 0;
     }
-    if (!spec->dll_name)  /* set default name from file name */
+    if (!spec->dll_name && spec->file_name)  /* set default name from file name */
     {
         char *p;
         spec->dll_name = xstrdup( spec->file_name );