ART: Don't leave rex_ & length_ uninitialized

Don't leave fields uninitialized in the x86-64 utils assembler.

Bug: 16993295

(cherry picked from commit 181211de89df5615395a55114fe551b08e86bc4d)

Change-Id: I2d648991fc2a6a4d5f518eb9ef0bb10857f6318a
1 file changed