libdraw, acme: fix acme segfault triggered passing an invalid font

acme -f nosuchfont

Change-Id: Iaa727db02b43e63082130796ec97c0efb7fe2b19
Reviewed-on: https://plan9port-review.googlesource.com/1220
Reviewed-by: Russ Cox <rsc@swtch.com>

src/libdraw/openfont.c

diff --git a/src/libdraw/openfont.c b/src/libdraw/openfont.c
index f798983..fdbc656 100644
--- a/src/libdraw/openfont.c
+++ b/src/libdraw/openfont.c
@@ -221,6 +221,8 @@
 		*p = '\0';
 
 	f = openfont1(d, name);
+	if(!f)
+		return nil;
 	f->lodpi = f;
 	f->namespec = namespec;