[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
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFListTable.cpp b/llvm/lib/DebugInfo/DWARF/DWARFListTable.cpp
index 5f5f12a..2124a49 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFListTable.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFListTable.cpp
@@ -29,13 +29,13 @@
   uint8_t OffsetByteSize = Format == dwarf::DWARF64 ? 8 : 4;
   uint64_t FullLength =
       HeaderData.Length + dwarf::getUnitLengthFieldByteSize(Format);
-  assert(FullLength == length());
   if (FullLength < getHeaderSize(Format))
     return createStringError(errc::invalid_argument,
                        "%s table at offset 0x%" PRIx64
                        " has too small length (0x%" PRIx64
                        ") to contain a complete header",
                        SectionName.data(), HeaderOffset, FullLength);
+  assert(FullLength == length() && "Inconsistent calculation of length.");
   uint64_t End = HeaderOffset + FullLength;
   if (!Data.isValidOffsetForDataOfSize(HeaderOffset, FullLength))
     return createStringError(errc::invalid_argument,