[DebugInfo] Fix a possible crash when reading a malformed .debug_*lists section.

DWARFListTableHeader::length() handles the zero value of HeaderData.Length
in a special way, which makes the result different from the calculated
value of FullLength, which leads to triggering an assertion. The patch
moves the assertion a bit later when `FullLength` is already checked for
minimal allowed value.

Differential Revision: https://reviews.llvm.org/D82886