The original implementation had the following comment:
// Ignore mv costing if mvsadcost is NULL

However the current implementation does not allow for this.
If x exists then nmvsadcost must not be null.

This removes the only warning from -Wpointer-bool-conversion

https://code.google.com/p/webm/issues/detail?id=894

Change-Id: I1a2cee340d7972d41e1bbbe1ec8dfbe917667085