Fix 5e413c9d: Last sprite offset in GRF file was not recorded (#10062)

A line was missed when preparing PR #9988, as this had been added
in a commit prior to the one which the PR was immediately based on

src/spritecache.cpp

@@ -592,6 +592,7 @@ void ReadGRFSpriteOffsets(SpriteFile &file)
 			}
 			file.SkipBytes(length);
 		}
+		if (prev_id != 0) _grf_sprite_offsets[prev_id] = offset;
 
 		/* Continue processing the data section. */
 		file.SeekTo(old_pos, SEEK_SET);