Fix RIOTest.testZeroLengthPrefix breakage on 3.10

Between Linux 3.12 and 3.13, handling of RIOs with a zero length prefix
changed. This patch updates RIOTest.testZeroLengthPrefix to document and
check for the alternate behavior in kernels older than 3.13.

Bug: 33333670
Test: Ran net_tests against android-3.10, 3.18, 4.1, 4.4
Change-Id: I0c3dd64075a50a19ff6f26e552af4a836bf655f9