Fix: syscalls: address of statically allocated element never null

This check is intended to confirm that the table element for that syscall
is indeed populated but checked that the element is NULL. This was never
the case because the address of an element of a statically allocated
array cannot be NULL.

Fix this by check if the function pointer is NULL instead. This means
that the element is not populated.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I1d769d6609fa4517199f022e1a262c4494c8f63a