Fix a leak for the subfont when using other fonts than the default.

Notes:
Thu May 26 08:01:02 EDT 2005 rsc
    have to think about this.  you've correctly identified the
    cause of the leak, i just need to figure out whether
    this is the right fix.
    

Sat May 28 08:01:49 EDT 2005 rsc
    I believe that there is a leak, but I do not believe
    that this change can possibly fix it.  It may well
    stir things around to make it harder to find.
    
    Do you have a simple program that demonstrates the leak?
    


Reference: /n/sources/patch/sorry/stringwidth2
Date: Thu May 26 13:36:13 CES 2005
Reviewed-by: rsc

--- /sys/src/libdraw/string.c	Thu May 26 13:36:04 2005
+++ /sys/src/libdraw/string.c	Thu May 26 13:36:01 2005
@@ -65,6 +65,7 @@
 	char **sptr;
 	Rune **rptr;
 	Font *def;
+	Subfont* sf;
 
 	if(s == nil){
 		s = "";
@@ -124,13 +125,14 @@
 			len -= n;
 		}
 		if(subfontname){
-			if(_getsubfont(f->display, subfontname) == 0){
+			if((sf=_getsubfont(f->display, subfontname)) == 0){
 				def = f->display->defaultfont;
 				if(def && f!=def)
 					f = def;
 				else
 					break;
-			}
+			} else
+				installsubfont(subfontname, sf);
 		}
 	}
 	return pt;