fontsrv: do not try to look up surrogate pairs

Change-Id: Ib0a680e123ba02b41e1029e356543c24e8b4647e
Reviewed-on: https://plan9port-review.googlesource.com/1430
Reviewed-by: Russ Cox <rsc@swtch.com>

src/cmd/fontsrv/osx.c

@@ -26,6 +26,8 @@ mapUnicode(int i)
 	case '`':
 		return 0x2018;
 	}
+	if(0xd800 <= i && i < 0xe0000) // surrogate pairs, will crash OS X libraries!
+		return 0xfffd;
 	return i;
 }