Running freewrl on a recent Gentoo Linux system with fontconfig-2.9.0 installed results in the following:
~ $ freewrl
opengl version=2.1 Mesa 8.1-devel
freewrl: fcmatch.c:548: IA__FcFontMatch: Assertion `result != ((void *)0)' failed.
That's from the call FcFontMatch(0,FW_fp,0) from FW_make_fontname() in src/lib/scenegraph/Component_Text.c due to the third parameter being 0.