logging: Fix RTT backend drop mode and coverity issue

Drop mode in RTT backend was broken. It is not the default
one so most likely it was unnoticed for a long time. Fixed
to report drop message when logs are lost.

Additionally, added assert to ensure that memmove does not
do memory overwrite.

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