Fix a crash when the right-side diff doesn't have a header.

There was an error when parsing a diff where there were headers on the
left-hand side but not the right-hand side. The solution was found by
Ze Lin Xiao at NetApp.

I couldn't devise a repro case, unfortunately. However, it appears to have
fixed the problem at NetApp, and logically makes sense. We'll need more
real-world testing.