venti/copy: fix bug writing directories that zero truncate

Found by nwf.

TBR=rsc
https://codereview.appspot.com/162860045

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 36522a1..0b9a541 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -37,6 +37,7 @@
 Mathieu Lonjaret <mathieu.lonjaret@gmail.com>
 Michael Teichgräber <mt4swm@googlemail.com>
 Michael Teichgräber <mt@ib.wmipf.de>
+Nathaniel W Filardo <nwf@cs.jhu.edu> <nwfilardo@gmail.com>
 Nikolai Saoukh <nikolai.saoukh@gmail.com>
 Yuval Pavel Zholkover <paulzhol@gmail.com>
 Peter Saveliev <svinota.saveliev@gmail.com>

src/cmd/venti/copy.c

diff --git a/src/cmd/venti/copy.c b/src/cmd/venti/copy.c
index 8c791f2..d6d770b 100644
--- a/src/cmd/venti/copy.c
+++ b/src/cmd/venti/copy.c
@@ -136,7 +136,7 @@
 		break;
 
 	case VtDirType:
-		for(i=0; i<n/VtEntrySize; i++){
+		for(i=0; i*VtEntrySize < n; i++){
 			if(vtentryunpack(&e, buf, i) < 0){
 				fprint(2, "warning: could not unpack entry #%d in %V %d\n", i, score, type);
 				continue;