sys: ring_buffer: fix possible ring_buf_put_claim/get_claim wrong size

- The issue is caused by the MIN() macro, which expands to
  (a)<(b)?(a):(b), where ring_buf_space_get()/ring_buf_size_get()
  is used as 'b' and is evaluated twice. The issue occurs when
  the (a)<(b) condition evaluates such that (b) is selected,
  but the value of (b) changes between evaluations, resulting
  in a possibly larger value than (a).
- Fixes the potential incorrect behavior by storing the result
  of ring_buf_space_get()/ring_buf_size_get() in a variable
  before using it in the MIN macro.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>