logging: Fix tracking of active messages

A variable is tracking number of buffered messages. This is used
to trigger processing thread in certain cases. Counter was not
handled correctly when message was dropped. In certain cases that
can lead to hanging of log processing.

Added counter decrementation in the callback called whenever
message is dropped.

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