lib: os: cbprintf_nano: Fix Coverity issue 316025

Coverity does not like that we are passing a pointer to a location
just beyond fixed array. Inside the function access is done through
negative indexes so there was no memory corruption but to satisfy
Coverity pointer to the last element of the array is passed and
we start from index 0 instead of -1.

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