[libunwind] Fix evaluating DWARF operation DW_OP_pick

reg is unsigned type and used here for getting array element from the end by
negating it. negation of unsigned can result in large number and array access
with that index will result in segmentation fault.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=43872

Patched by: kamlesh kumar

Differential Revision: https://reviews.llvm.org/D69893