fix warnings treated as errors, caused by using CFLAGS -Wall -Werror

I came across these errors while building varnish 4.0.0, having subtree-merged libvmod-digest into the varnish tree; the latter uses CFLAGS -Wall -Werror by default.

I have barely read the code, there are probably better ways to fix these warnings, but I just wanted to let you know about them.