Fix: backported from urcu commit 92af1a30

Fix: backported from urcu commit 92af1a30

commit 92af1a30ca6a70945b167c31631c8598a626c71a
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date:   Thu Nov 28 18:41:13 2013 +0100

    Fix undefined NULL pointer arithmetic

    Clang 3.3 with -O2 optimisations is especially picky about arithmetic
    on NULL pointers. This undefined behavior is turned into optimized out
    NULL checks by clang 3.3. Fix the undefined behavior by checking against
    the pointer directly, without going back and forth around NULL with
    pointer arithmetic.

    Reported-by: Zifei Tong <soariez@gmail.com>
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>