Avoid LLDB crash upon DW_OP_deref* with empty stack
    
As done in other DW_OP_* cases, return an error if the stack is empty
rather than eventually crashing elsewhere.  Encountered on big-endian
MIPS, where LLVM bugs currently result in invalid .debug_loc data.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@199110 91177308-0d34-0410-b5e6-96231b3b80d8