gcc on arm64 -Og produces a (false positive) warning that
stackPos might be used uninitialised.

Silence gcc by assigning a value to stackPos.
This value must be overriden if a stack description is found.
The fact that the value is overriden is asserted.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14438 a5019735-40e9-0310-863c-91ae7b9d1cf9