[ASan][MIPS] Fix null_deref.cc test on MIPS64

Symbol for pc used in ucontext->uc_mcontext was wrong.

Reviewers: dsanders, kcc, samsonov
Subscribers: llvm-commits, mohit.bhakkad, jaydeep
Differential Revision: http://reviews.llvm.org/D9246

llvm-svn: 236572