Created attachment 334624 [details]
Created attachment 334626 [details]
Two dependencies (net-libs/libpcap-1.3.0 and something else) downloaded and installed fine, but I've been having the same problem with other emerge targets as well: dev-vcs/git-220.127.116.11 and app-shells/bash-4.2_p37 (which is a dependency for emergeing portage). The problem may be larger than just nmap.
It looks like there might be something wrong with the filesystem that you store the distfiles on. The downloads look identical in size, so it can only be the way they are stored locally. Sadly, we can't fix that for you.