Subzero: Fix a crash in mem operand dumping.
Subzero: Fix a crash in mem operand dumping.

getOffset() was being dereferenced even when it was nullptr.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1435283003 .

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader