Fix 343173 - helgrind crash during stack unwind

Fix 343173 - helgrind crash during stack unwind

This fixes a helgrind crash detected on android.
Android bionic pthread lib unmaps the stack for detached threads
before exiting.
Helgrind tries to unwind the stack to record a 'read' after
the stack unmap, just before the exit syscall.
The unwind then causes a SEGV.

The solution consists in tightening the calculation of
the stack limits, so as to stop unwinding when no valid stack
can be found.
Regression test reproduces the same problem by simulating the
bionic behaviour on linux, using asm similar to bionic lib.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14976