Fix 'refd != NULL' assertion in write_die

[ Backport of master commits 6959430 and 7cc8aae. ]

When running dwz on a file that contains invalid DW_FORM_ref_addr attributes
(which has been observed to be generated by a google go compiler) we run
either into an assert:
...
$ dwz multidictionary
dwz: dwz.c:9461: write_die: Assertion `refd != NULL' failed.
Aborted (core dumped)
...
or a segmentation fault in case of low-mem mode:
...
$ dwz -l0 multidictionary
Segmentation fault (core dumped)
...

Fix this by erroring out instead:
...
$ dwz multidictionary
dwz: Couldn't find DIE at DW_FORM_ref_addr offset 0x97
...

2019-02-05  Tom de Vries  <tdevries@suse.de>

	PR dwz/24169
	* dwz.c (write_die): Error out on invalid DW_FORM_ref_addr.

---
 dwz.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/dwz.c b/dwz.c
index 928fefa..c7db337 100644
--- a/dwz.c
+++ b/dwz.c
@@ -28,6 +28,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <inttypes.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <sys/mman.h>
@@ -9124,6 +9125,9 @@ write_die (unsigned char *ptr, dw_cu_ref cu, dw_die_ref die,
 					  ? ptr_size : 4);
 		inptr += refcu->cu_version == 2 ? ptr_size : 4;
 		refd = off_htab_lookup (NULL, value);
+		if (refd == NULL || refd->die_tag == 0)
+		  error (1, 0, "Couldn't find DIE at DW_FORM_ref_addr offset"
+			 " 0x%" PRIx64, value);
 		assert (refd != NULL);
 		refdt = refd;
 		while (refdt->die_toplevel == 0)