Handle invalid tree extensions

Handle invalid tree extensions

In case the index contains wrong tree extensions don't throw a
ArrayIndexOutOfBounds exception but revalidate the tree extension.

It happened that the git index written by Git for Windows contained valid
(means entryCount>0) tree extensions for pathes which are not existing
in the index. Native git handles this inconsistency silently but JGit
was crashing with a ArrayIndexOutOfBounds exception. Teach JGit to
better recognize such cases and revalidate such extensions.

It's hard to write a test because JGit doesn't write such extensions. It
only reads, validates and makes use of them. But the bug tells how to
create such situations.

Bug: 457152
Change-Id: Id3ffd7dc7ae1c55674d88bf1b43953234fe0b68d