Cleanup: Avoid possible NULL pointer error

In normal conditions, `gpf` always has a value, but better move inside the NULL checking.

source/blender/editors/gpencil/editaction_gpencil.c

@@ -454,9 +454,10 @@ bool ED_gpencil_anim_copybuf_paste(bAnimContext *ac, const short offset_mode)
 
       /* get frame to copy data into (if no frame returned, then just ignore) */
       gpf = BKE_gpencil_layer_frame_get(gpld, gpfs->framenum, GP_GETFRAME_ADD_NEW);
-      /* Ensure to use same keyframe type. */
-      gpf->key_type = gpfs->key_type;
       if (gpf) {
+        /* Ensure to use same keyframe type. */
+        gpf->key_type = gpfs->key_type;
+
         bGPDstroke *gps, *gpsn;
 
         /* This should be the right frame... as it may be a pre-existing frame,