[LLDB] Fix buffer overflow problem in DWARFExpression::Evaluate

In two calls to ReadMemory in DWARFExpression.cpp, the buffer size
passed to ReadMemory is not checked and can be bigger than the actual
size of the buffer. This caused a buffer overflow bug, which we
found through Address Sanitizer. This patch fixes the problem by
checking the address size when it is first read out of the DWARF, and
setting an error and returning immediatley if the size is invalid.

This is the second attempt to fix this issue; I reverted the first one,
as it was not quite correct.

Differential Revision: https://reviews.llvm.org/D154907