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
diff --git a/compiler/utils/x86_64/assembler_x86_64.h b/compiler/utils/x86_64/assembler_x86_64.h
index 2f814df..9a6df1c 100644
--- a/compiler/utils/x86_64/assembler_x86_64.h
+++ b/compiler/utils/x86_64/assembler_x86_64.h
@@ -146,7 +146,7 @@
   uint8_t length_;
   uint8_t encoding_[6];
 
-  explicit Operand(CpuRegister reg) { SetModRM(3, reg); }
+  explicit Operand(CpuRegister reg) : rex_(0), length_(0) { SetModRM(3, reg); }
 
   // Get the operand encoding byte at the given index.
   uint8_t encoding_at(int index) const {