Avoid bogus internal error reporting duplicate In-Reply-To IDs.

This error was tirggered with a debugging build via:

	make CXXFLAGS="-DDEBUG"

and reported by David Bremner. The actual error is that I'm an
idiot that doesn't know how to use strcmp's return value. Of
course, the strcmp interface scores a negative 7 on Rusty Russell
ranking of bad interfaces:

http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html