logging: When strings are stripped keep source name pointers

90ab94f6 added a change that source name pointers were not stored
in the const data structure associated with a logging source. That was
done because those string pointers were invalid (on purpose pointing to
non-existing memory) and reading those strings could lead to a fault.
However, those pointers are used by scripts which are building the
dictionary database and after this change script was no longer
able to retrieve source names from elf file and database was incomplete.

This patch brings back storing of source name string pointers but in
API for reading those source names guards are added to not return
invalid addresses.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>