drmSL: Fix neighbor lookup

Commit e4a51963:
    Tidy up compile warnings by cleaning up types.

removed call to SLLocate which gutted the function of all functionality.
This patch restores the original behavior, with an additional fix
that zeros the update array in case SLLocate bails early.

v2: zero the update array instead of checking the return value.
    SLLocate returns NULL both on failure and if the element is greater
    than everything in the list
v3: Improve commit message

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>