Remove check for an impossible condition

offset_for_ref_frame is an array rather than a pointer - it can't be
NULL (and the check causes a warning in clang).

Change-Id: I9b96e1e723e3b8caebd41821137ed6fb669d8cd4
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>