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