logging: log_core: correct timeout of -1 ms to K_FOREVER

Many releases ago, specifying to block indefinitely in the log
processing thread would do just that.

However, a subtle bug was introduced  such that specifying -1
for `CONFIG_LOG_BLOCK_IN_THREAD_TIMEOUT_MS` would have the
exact opposite effect than what was intended.

As per Kconfig, a value of -1 should translate to a timeout of
`K_FOREVER`. However, conversion via `K_MSEC(-1)` results in
a `k_timeout_t` that is equal to `K_NO_WAIT` rather than the
intent which is `K_FOREVER`.

Add a dedicated check to to ensure that a value of -1 is
correctly interpreted as `K_FOREVER` in `log_core.c`.

For reference, the blocking feature was described in #15196,
added in #16194, and it would appear that the regression
happened in c5f2cdef.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>