Caused by rBf3bf87e5887c: GPencil: Small frame evaluation speed up improvement.

When using a GPencil Time Offset Modifier, the bGPDlayer>actframe can be
NULL. This can be determined though, but above optimization commit
skipped getting the active frame in this case entirely (with the
intention to only get it if framenumbers did not match).

Now also call BKE_gpencil_layer_frame_get() if actframe is NULL in order
to fetch a valid one if present.