[LSAN] Fix test swapcontext.cc on MIPS

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.

Reviewed by aizatsky.
Differential: http://reviews.llvm.org/D19961

llvm-svn: 269882