[LLD] [COFF] Fix crashes for cfguard with undefined weak symbols (#79063)

When marking symbols as having their address taken, we can have the
sitaution where we have the address taken of a weak symbol. If there's
no strong definition of the symbol, the symbol ends up as an absolute
symbol with the value null. In those cases, we don't have any Chunk.
Skip such symbols from the cfguard tables.

This fixes https://github.com/llvm/llvm-project/issues/78619.