Fix "ninja check-lldb" crash in IRExecutionUnit.cpp

Summary:
From Adrian McCarthy:

"Running ninja check-lldb now has one crash in a Python process, due to deferencing a null pointer in IRExecutionUnit.cpp:  candidate_sc.symbol is null, which leads to a call with a null this pointer."

Reviewers: zturner, spyffe, amccarth

Subscribers: ted, jingham, lldb-commits

Differential Revision: http://reviews.llvm.org/D17860

llvm-svn: 263066