Subzero: Fix a potential null-pointer dereference.
Subzero: Fix a potential null-pointer dereference.

BUG= none
R=jpp@chromium.org, manasijm@google.com

Review URL: https://codereview.chromium.org/2103613002 .

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