lib: os: spsc_pbuf: Fix race condition

Allocator is adding padding in 2 steps. First padding mark is written
to the buffer and then write index is reset to 0. Consumer may interrupt
this operation (another thread or core) and fail since it was only checking
padding marker and that alone was enough to consume the padding. If that
happen before write index got updated, buffer reading become corrupted.
Fixing it by adding write index check when padding is found.

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