Fix double free in load_file_object

unload_datafile already calls _AL_FREE.
diff -up allegro-4.4.3/src/datafile.c~ allegro-4.4.3/src/datafile.c
--- allegro-4.4.3/src/datafile.c~	2019-02-25 23:33:13.000000000 +0100
+++ allegro-4.4.3/src/datafile.c	2019-02-25 23:39:39.320622137 +0100
@@ -1348,7 +1348,6 @@ static void *load_file_object(PACKFILE *
    /* gracefully handle failure */
    if (failed) {
       unload_datafile(dat);
-      _AL_FREE(dat);
       dat = NULL;
    }