freefont() can suicide if you have not called initdraw() - not unresaonable you may think,
but some programs, notably mc(1) want to know the size of their window but have no interest
in drawing anything. mc(1) doen't bother calling freefont(), it just exits but my
application is long lived and so mus not leak, hence this patch.

-Steve

Reference: /n/sources/patch/applied/freefont-buglet
Date: Mon Jul 25 18:34:04 CES 2011
Signed-off-by: steve@quintile.net

--- /sys/src/libdraw/buildfont.c	Mon Jul 25 18:28:27 2011
+++ /sys/src/libdraw/buildfont.c	Mon Jul 25 18:28:24 2011
@@ -129,7 +129,7 @@
 	}
 	for(i=0; i<f->nsubf; i++){
 		s = f->subf[i].f;
-		if(s && s!=display->defaultsubfont)
+		if(s && display && s!=display->defaultsubfont)
 			freesubfont(s);
 	}
 	freeimage(f->cacheimage);