split-index: BUG() when cache entry refers to non-existing shared entry

split-index: BUG() when cache entry refers to non-existing shared entry

When the split index feature is in use, then a cache entry is:

  - either only present in the split index, in which case its 'index'
    field must be 0,

  - or it should refer to an existing entry in the shared index, i.e.
    the 'index' field can't be greater than the size of the shared
    index.

If a cache entry were to refer to a non-existing entry in the shared
index, then that's a sign of something being wrong in the index state,
either as a result of a bug in dealing with the split/shared index
entries, or perhaps a (potentially unrelated) memory corruption issue.

prepare_to_write_split_index() already has a condition to catch cache
entries with such bogus 'index' field, but instead of calling BUG() it
just sets cache entry's 'index = 0', and the entry will then be
written to the new split index.

Don't write a new index file from bogus index state, and call BUG()
upon encountering an cache entry referring to a non-existing shared
index entry.

Running the test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes'
doesn't trigger this condition.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>