Log: Fix build error on Fedora 30

Fedora 30 uses GCC 9.1 which fails to build when loggin is enabled:

zephyr/subsys/logging/log_core.c: In function ‘log_is_strdup’:
zephyr/subsys/logging/log_core.c:665:22:
warning: array subscript -1 is outside array bounds of
‘u8_t[]’ [-Warray-bounds]
  665 |         ((char *)buf <= pool_last->buf);
      |                      ^~
zephyr/subsys/logging/log_core.c:52:3:
note: while referencing ‘log_strdup_pool_buf’
   52 |   log_strdup_pool_buf[LOG_STRDUP_POOL_BUFFER_SIZE];

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>