verity: don't error on depth=0

Support for depth=0 (i.e. compute depth) was added as part of:

commit 8019a6a4fdc022430376e270cdcc2b19113527c8
Author: Mandeep Singh Baines <msb@chromium.org>
Date:   Fri Apr 1 15:07:39 2011 -0700

    verity: create a regular trie when depth=0

BUG=9752
TEST=Ran dm-verity.git unit tests. Ran platform_DMVerityCorruption on H/W.

Change-Id: Ie5ecaf8c8bb0e06e80edb9bad3170dc14d0983c2

R=wad@chromium.org,taysom@chromium.org,ups@chromium.org

Review URL: http://codereview.chromium.org/6810004
diff --git a/file_hasher.cc b/file_hasher.cc
index fd6bda2..14e9604 100644
--- a/file_hasher.cc
+++ b/file_hasher.cc
@@ -48,7 +48,7 @@
                             unsigned int depth,
                             unsigned int blocks,
                             const char *alg) {
-  if (depth == 0 || !alg || !source || !destination) {
+  if (!alg || !source || !destination) {
      LOG(ERROR) << "Invalid arguments supplied to Initialize";
      LOG(INFO) << "depth: " << depth;
      LOG(INFO) << "s: " << source << " d: " << destination;