[LSAN] Fix test swapcontext.cc on MIPS

Summary: There is no frame validity check in the slow unwinder like there is in the fast unwinder due to which lsan reports a leak even for heap allocated coroutine in the test swapcontext.cc. Since mips/linux uses slow unwindwer instead of fast unwinder, the test fails for mips/linux. Therefore adding the checks before unwinding fixes the test for mips/linux.

Reviewers: samsonov, earthdok, kcc
Subscribers: llvm-commits, mohit.bhakkad, jaydeep
Differential: http://reviews.llvm.org/D18690
llvm-svn: 266716