lib: ring_buf: Fix unused-but-set compilation warnings

For the error codes used only in __ASSERT() statements fix compilation
warnings like:

...
warning: variable 'err' set but not used [-Wunused-but-set-variable]
        int err;
            ^
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
1 file changed