Fix attempt to free already freed memory when loading an animation

in the sequencer, was in the code that tests if it's a file that it
understands. Didn't cause a crash here though, just warning.

source/blender/imbuf/intern/anim.c

@@ -1060,7 +1060,9 @@ struct ImBuf * IMB_anim_absolute(struct anim * anim, int position) {
 		if (ibuf == NULL) {
 			return (0);
 		}
+
 		IMB_freeImBuf(ibuf); /* ???? */
+		ibuf= NULL;
 	}
 
 	if (position < 0) return(0);