Fix ObjectWalk corruption when skipping over empty trees

Fix ObjectWalk corruption when skipping over empty trees

The supplied test case comes out of the example tree identified by
Robert de Wilde and Ilari on #git:

  $ git ls-tree -rt a54f1a85ebf6a7f53aa60a45a1be33f8b078fb7e
  040000 tree bfe058ad536cdb12e127cde63b01472c960ea105    A
  040000 tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904    A/A
  040000 tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904    A/B
  100644 blob abbbfafe3129f85747aba7bfac992af77134c607    B

In this tree, "B" was being skipped because "A/A" as an empty tree
was immediately followed by "A/B", also an empty tree, but the
ObjectWalk broke out too early and never visited "B".

Bug: 286653
Change-Id: I25bcb0bc99d0cbbbdd9c2bd625ad6a691a6d0335
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>