QFontEngine_FT: Don't check for null after dereferencing.

Must have been a copy paste mistake, the check is used in other places
to make sure we don't unlockFace() without a lockFace(), but here
we're certain lockFace() was called first.

Makes Coverity happy.

Change-Id: I679129727b29a40d780f4fa903f44f7cbc9ec8cf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>