linke_message: Avoid segfault when In-Reply-to header is empty.

This was recently introduced in commit:

	64c03ae97f2f5294c60ef25d7f41849864e6ebd3

which was adding extra checks to avoid adding a self-referencing
message.

How many times am I going to fix a dumb regression like this and say
"we really need a test suite" before I actually sit down and write the
test suite?