libdw: Handle NULL dbg for call_ref, GNU_implicit_pointer and addr DW_OPs.

libdw: Handle NULL dbg for call_ref, GNU_implicit_pointer and addr DW_OPs.

afl-fuzz pointed out that __libdw_intern_expression didn't handle CFI
containing DW_OP_call_ref, DW_OP_implicit_pointer or DW_OP_addr. Because
in that case the Dwarf dbg is NULL. Both DW_OP_call_ref and
DW_OP_implicit_pointer cannot be used in CFI. That is just an error.
But DW_OP_addr can be. Without a Dwarf dbg we'll need to read the address
argument directly. Don't use __libdw_read_address_inc which might do a
relocation of the value read. But in practice the relocation hook isn't
implemented anyway.

Signed-off-by: Mark Wielaard <mjw@redhat.com>