vbackup: fix uninitialized variable use in -i code

src/cmd/vbackup/vbackup.c

diff --git a/src/cmd/vbackup/vbackup.c b/src/cmd/vbackup/vbackup.c
index a80a2c2..07000b1 100644
--- a/src/cmd/vbackup/vbackup.c
+++ b/src/cmd/vbackup/vbackup.c
@@ -239,6 +239,7 @@
 			sysfatal("file system block counts don't match %lld %lld", e.size, fsys->nblock*bsize);
 	}
 
+	tmpnam = nil;
 	if(incremental){
 		if(vtfilegetentry(vfile, &e) < 0)
 			sysfatal("vtfilegetentry: %r");