mem: fix potential resource leak on freeing

We close fd if we managed to find it in the list of allocated
segment lists (which should always be the case under normal
conditions), but if we didn't, the fd was leaking. Close it if
we couldn't find it in the segment list. This is not an issue
as if the segment is zero length, we're getting rid of it
anyway, so there's no harm in not storing the fd anywhere.

Coverity issue: 272568
Fixes: 2a04139f66b4 ("eal: add single file segments option")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>