safer string equality in cache
safer string equality in cache

it could segv if operations had strings which had been set to NULL

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