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ndb/dns: fix nil dereference crash with convM2DNS() returning reqmsg.qd == nil

--- a/sys/src/cmd/ndb/dnstcp.c

+++ b/sys/src/cmd/ndb/dnstcp.c

@@ -122,6 +122,12 @@

 					reqmsg.flags & Omask, caller);

 				break;

 			}

+

+		if(reqmsg.qd == nil){

+			dnslog("server: no question RR from %s", caller);

+			break;

+		}

+

 		if(debug)

 			dnslog("[%d] %d: serve (%s) %d %s %s",

 				getpid(), req.id, caller,

--- a/sys/src/cmd/ndb/dnudpserver.c

+++ b/sys/src/cmd/ndb/dnudpserver.c

@@ -215,6 +215,11 @@

 			goto freereq;

 		}

+		if(reqmsg.qd == nil){

+			dnslog("server: no question RR from %I", buf);

+			goto freereq;

+		}

+

 		if(debug || (trace && subsume(trace, reqmsg.qd->owner->name)))

 			dnslog("%d: serve (%I/%d) %d %s %s",

 				req.id, buf, uh->rport[0]<<8 | uh->rport[1],