net: socketpair: Fix use after free

In low memory conditions, its possible for socketpair memory allocation
to fail and then the socketpair is freed but after that the remote
semaphore is released causing a crash.

Fix this by freeing the socketpair after releasing the semaphore. Add a
test case to induce low memory conditions (low HEAP and high socketpair
buffer size), with the fix issue is not seen.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>