devel/gdb: Avoid a divide by zero in kgdb.

Don't divide by zero if the message buffer size in a crashdump is
zero.

Reported by:	glebius
Reviewed by:	pizzamig (maintainer), emaste
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D32486

devel/gdb/Makefile

@@ -2,6 +2,7 @@
 
 PORTNAME=	gdb
 DISTVERSION=	11.1
+PORTREVISION=	1
 CATEGORIES=	devel
 MASTER_SITES=	GNU
 

devel/gdb/files/kgdb/fbsd-kvm.c

@@ -144,6 +144,8 @@ kgdb_dmesg(void)
 	} catch (const gdb_exception_error &e) {
 		return;
 	}
+	if (size == 0)
+		return;
 	rseq = MSGBUF_SEQ_TO_POS(size, rseq);
 	wseq = MSGBUF_SEQ_TO_POS(size, wseq);
 	if (rseq == wseq)