revise libjpeg new from buffer
revise libjpeg new from buffer

in line with latest libjpeg recommendations

this fixes a segv with corrupt input found by libFuzzer and asan

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