Add assert in partition_found_dups

With the reproducer from PR26252 we get:
...
$ dwz -m 3 --odr 1 2
dwz: 1: DWARF compression not beneficial - old size 317760 new size 317760
dwz: dwz.c:12431: write_die: \
  Assertion `value && refdcu->cu_kind != CU_ALT' failed.
Aborted (core dumped)
...

The assertion fails when writing out the contribution of file 2 to the
multifile.

It's trying to write out a copy of this DIE:
...
 <3><9f3e>: Abbrev Number: 149 (DW_TAG_subprogram)
    <9f40>   DW_AT_name        :
             _M_access<ydotool::Tool::ToolManager::ScanPath(const string&)::\
	       <lambda(const string&, dirent*)> >
    <9f44>   DW_AT_decl_file   : 11
    <9f45>   DW_AT_decl_line   : 92
    <9f46>   DW_AT_decl_column : 7
    <9f47>   DW_AT_type        : <0x1a2a5>
    <9f4b>   DW_AT_declaration : 1
    <9f4b>   DW_AT_object_pointer: <0x9f5c>
    <9f4f>   DW_AT_sibling     : <0x9f62>
...
and specifically, for the DW_AT_type attribute it attempts to write out the
reference to the copy of the DIE at 0x1a2a5:
...
 <1><1a2a5>: Abbrev Number: 9 (DW_TAG_reference_type)
    <1a2a6>   DW_AT_byte_size   : 8
    <1a2a7>   DW_AT_type        : <0x27cd9>
...

There is no copy of that DIE, because it has a bad checksum:
...
  1a2a5 X d27b2f2a d27b2f2a  reference_type (type: 27cd9 structure_type)
...
which means that its toplevel DIE 0x9edc:
...
<2><9edc>: Abbrev Number: 182 (DW_TAG_union_type)
    <9ede>   DW_AT_name        : _Any_data
...
should also have a bad checksum, but it doesn't:
...
   9edc O 9d6524b4 9d6524b4 _Any_data union_type
...
while without --odr, it does:
...
   9edc X d8f947f3 5100c9e1 _Any_data union_type
...

Add an assert partition_found_dups such that we can catch the problem of the
checksum much earlier:
...
 static void
 partition_found_dups (dw_die_ref die, struct obstack *vec)
 {
+  assert (die->die_ck_state == CK_KNOWN);
...
which means we can reproduce with just one file:
...
$ dwz 2 --odr
dwz: dwz.c:7372: partition_found_dups: \
  Assertion `die->die_ck_state == CK_KNOWN' failed.
Aborted (core dumped)
...

2021-02-18  Tom de Vries  <tdevries@suse.de>

	* dwz.c (partition_found_dups): Assert that
	die->die_ck_state == CK_KNOWN.

---
 dwz.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/dwz.c b/dwz.c
index d037e1e..0b751fc 100644
--- a/dwz.c
+++ b/dwz.c
@@ -7393,6 +7393,7 @@ partition_cmp (const void *p, const void *q)
 static void
 partition_found_dups (dw_die_ref die, struct obstack *vec)
 {
+  assert (die->die_ck_state == CK_KNOWN);
   obstack_ptr_grow (vec, die);
   if (unlikely (verify_dups_p))
     verify_dups (die, true);