fix memleak in exif write
fix memleak in exif write

jcupitt/libvips,mcanthony/libvips,jcupitt/libvips,mcanthony/libvips,wishabi/libvips,lovell/libvips,wishabi/libvips,leiyangyou/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,leiyangyou/libvips,lovell/libvips,wishabi/libvips,mcanthony/libvips,leiyangyou/libvips,wishabi/libvips,leiyangyou/libvips,mcanthony/libvips,jcupitt/libvips