selftests/bpf: Remove memory leak

The allocated entry is immediately overwritten by an assignment. Fix
that.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210122154725.22140-5-bjorn.topel@gmail.com