venti: fix possible nil deref in libventi vtgetreq() and cleanup vacfs (import from sources)

sys/src/cmd/vac/file.c

@@ -1592,6 +1592,11 @@ vacfilesetdir(VacFile *f, VacDir *dir)
 		f->dir.gid = vtstrdup(dir->gid);
 	}
 
+	if(strcmp(f->dir.mid, dir->mid) != 0){
+		vtfree(f->dir.mid);
+		f->dir.mid = vtstrdup(dir->mid);
+	}
+
 	f->dir.mtime = dir->mtime;
 	f->dir.atime = dir->atime;
 
@@ -1774,7 +1779,7 @@ vacfsopen(VtConn *z, char *file, int mode, int ncache)
 	char *prefix;
 	
 	if(vtparsescore(file, &prefix, score) >= 0){
-		if(strcmp(prefix, "vac") != 0){
+		if(prefix == nil || strcmp(prefix, "vac") != 0){
 			werrstr("not a vac file");
 			return nil;
 		}

sys/src/cmd/vac/vacfs.c

@@ -200,12 +200,6 @@ threadmain(int argc, char *argv[])
 		mfd[0] = p[0];
 		mfd[1] = p[0];
 		srvfd = p[1];
-	}
-
-	procrfork(srv, 0, Stacksize, RFFDG|RFNAMEG|RFNOTEG);
-
-	if(!stdio){
-		close(p[0]);
 		if(defsrv){
 			srvname = smprint("/srv/%s", defsrv);
 			fd = create(srvname, OWRITE|ORCLOSE, 0666);
@@ -215,6 +209,12 @@ threadmain(int argc, char *argv[])
 				sysfatal("write %s: %r", srvname);
 			free(srvname);
 		}
+	}
+
+	procrfork(srv, 0, Stacksize, RFFDG|RFNAMEG|RFNOTEG);
+
+	if(!stdio){
+		close(p[0]);
 		if(defmnt){
 			if(mount(srvfd, -1, defmnt, MREPL|MCREATE, "") < 0)
 				sysfatal("mount %s: %r", defmnt);

sys/src/libventi/server.c

@@ -180,7 +180,9 @@ vtgetreq(VtSrv *srv)
 	VtReq *r;
 	
 	r = _vtqrecv(srv->q);
-	vtlog(VtServerLog, "<font size=-1>%T %s:</font> vtgetreq %F<br>\n", ((VtSconn*)r->sc)->c->addr, &r->tx);
+	if (r != nil)
+		vtlog(VtServerLog, "<font size=-1>%T %s:</font> vtgetreq %F<br>\n",
+			((VtSconn*)r->sc)->c->addr, &r->tx);
 	return r;
 }