odb: allow `odb_find_source()` to fail

odb: allow `odb_find_source()` to fail

When trying to locate a source for an unknown object directory we will
die right away. In subsequent patches we will add new callsites though
that want to handle this situation gracefully instead.

Refactor the function to return a `NULL` pointer if the source could not
be found and adapt the callsites to die instead. Introduce a new wrapper
`odb_find_source_or_die()` that continues to die in case the source
could not be found.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>