do_read_index(): always mark index as initialized unless erroring out

do_read_index(): always mark index as initialized unless erroring out

In 913e0e99b6a (unpack_trees(): protect the handcrafted in-core index
from read_cache(), 2008-08-23) a flag was introduced into the
`index_state` structure to indicate whether it had been initialized (or
more correctly: read and parsed).

There was one code path that was not handled, though: when the index
file does not yet exist (but the `must_exist` parameter is set to 0 to
indicate that that's okay). In this instance, Git wants to go forward
with a new, pristine Git index, almost as if the file had existed and
contained no index entries or extensions.

Since Git wants to handle this situation the same as if an "empty" Git
index file existed, let's set the `initialized` flag also in that case.

This is necessary to prepare for fixing the bug where the condition
`cache_nr == 0` is incorrectly used as an indicator that the index was
already read, and the condition `initialized != 0` needs to be used
instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>