Fix crash with recent refactor of customdata writing.

Caused by own rBff3d535bc2a6309 - since we now only write the exact amount of layers
needed to store saved customdata, we have to adjust CustomData->maxlayer too.

Otherwise, on next read, customdata code believes it has more layers allocated than
actual number.

Issue reported by Campbell over IRC, thanks.

source/blender/blenkernel/intern/customdata.c

@@ -3251,6 +3251,7 @@ void CustomData_file_write_prepare(
 		}
 	}
 	BLI_assert(j == data->totlayer);
+	data->maxlayer = data->totlayer;  /* We only write that much of data! */
 	*r_write_layers = write_layers;
 }