more stabilization

I had overlooked the fact that for a canceled request we might only read
parts of the body.

Interestingly, this is only exposed on the vtest ARMs.

Ref #2964

bin/varnishtest/tests/r02964.vtc

@@ -49,5 +49,5 @@ client c1 {
 
 logexpect l1 -wait
 
-varnish v1 -expect SM?.Transient.c_bytes > 131072
+varnish v1 -expect SM?.Transient.c_bytes > 0
 varnish v1 -expect SM?.Transient.g_bytes < 2000