try delaying libnsgif alloc until read frame
try delaying libnsgif alloc until read frame

gives the caller a chance to detect memory bombs

lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips