pull request #1007 - avoid free(emptystr)

Pull request from argrath:  save_luadata() sets 'lua_data' to
'emptystr' if the fetch variables routine returns Null, and later
passes 'lua_data' to free() unconditionally.  Freeing 'emptystr'
won't work and might crash (if the fetch routine ever returned Null,
which might never happen).

Fixes #1007

src/nhlua.c

@@ -1206,7 +1206,7 @@ save_luadata(NHFILE *nhfp)
     char *lua_data = get_nh_lua_variables(); /* note: '\0' terminated */
 
     if (!lua_data)
-        lua_data = emptystr;
+        lua_data = dupstr(emptystr);
     lua_data_len = Strlen(lua_data) + 1; /* +1: include the terminator */
     bwrite(nhfp->fd, (genericptr_t) &lua_data_len,
            (unsigned) sizeof lua_data_len);