Thanks for this patch, it's so useful (essential?), you wonder why it hasn't been incorporated into a new version of the code yet.  FYI, for it to work as is, you need to change the file name of the source to add ".orig".  I prefer to modify the first two lines to something like this:

--- mod_auth_xradius.c    2005-04-28 03:58:25.000000000 -0400
+++ mod_auth_xradius.c.PATCHED    2010-09-10 11:38:49.000000000 -0400

And run the patch command from within the src/ directory:

patch < patchfilename