libvpx

change to save rdmult value correctly

This commit moves the position where rdmult is saved to make sure it
is the correct value. Prior, an uninitialized value may be saved and
restored.

This addresses issue:
https://code.google.com/p/webm/issues/detail?id=733

Change-Id: I436407f289169bc63da3c5a6bf609bed16cb71b5