[lldb/DWARF] Fix evaluator crash when accessing empty stack.

This patch fixes a crash that happens on the DWARF expression evaluator
when trying to access the top of the stack while it's empty.

rdar://60512489

Differential Revision: https://reviews.llvm.org/D77108

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>