gossipd: fix crash in gossmap_fetch_tail.

It only gets called for diagnostics when something goes wrong (and we
were going to exit anyway), but it shouldn't crash:

```
**BROKEN** gossipd: Truncated gossmap record @7991501/7991523 (len 0): waiting
**BROKEN** gossipd: FATAL SIGNAL 6 (version v25.09)                                            
**BROKEN** gossipd: backtrace: common/daemon.c:41 (send_backtrace) 0x6506817cc529
**BROKEN** gossipd: backtrace: common/daemon.c:78 (crashdump) 0x6506817cc578
**BROKEN** gossipd: backtrace: ./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0 ((null)) 0x75e8267a032f
**BROKEN** gossipd: backtrace: ./nptl/pthread_kill.c:44 (__pthread_kill_implementation) 0x75e8267f9b2c
**BROKEN** gossipd: backtrace: ./nptl/pthread_kill.c:78 (__pthread_kill_internal) 0x75e8267f9b2c
**BROKEN** gossipd: backtrace: ./nptl/pthread_kill.c:89 (__GI___pthread_kill) 0x75e8267f9b2c
**BROKEN** gossipd: backtrace: ../sysdeps/posix/raise.c:26 (__GI_raise) 0x75e8267a027d
**BROKEN** gossipd: backtrace: ./stdlib/abort.c:79 (__GI_abort) 0x75e8267838fe
**BROKEN** gossipd: backtrace: ./assert/assert.c:96 (__assert_fail_base) 0x75e82678381a
**BROKEN** gossipd: backtrace: ./assert/assert.c:105 (__assert_fail) 0x75e826796516
**BROKEN** gossipd: backtrace: common/gossmap.c:111 (map_copy) 0x6506817cea77
**BROKEN** gossipd: backtrace: common/gossmap.c:1870 (gossmap_fetch_tail) 0x6506817d1f93
**BROKEN** gossipd: backtrace: gossipd/gossmap_manage.c:1442 (gossmap_manage_get_gossmap) 0x6506817c45fb
**BROKEN** gossipd: backtrace: gossipd/gossmap_manage.c:753 (gossmap_manage_handle_get_txout_reply) 0x6506817c5850
**BROKEN** gossipd: backtrace: gossipd/gossipd.c:574 (recv_req) 0x6506817c172b
```

Reported-by: @grubles
Fixes: #8542
Signed-off-by: Rusty Russell <[email protected]>
Changelog-Fixed: gossipd: crash on incoherent filesystems (e.g. ZFS on Linux)

Author: rustyrussell

common/gossmap.c

@@ -1868,10 +1868,9 @@ const u8 *gossmap_fetch_tail(const tal_t *ctx, const struct gossmap *map)
 	u8 *p;
 
 	/* Shouldn't happen... */
-	if (map->map_end > map->map_size)
+	if (map->map_end > map->map_size || !map->mmap)
 		return NULL;
 	len = map->map_size - map->map_end;
 	p = tal_arr(ctx, u8, len);
-	map_copy(map, map->map_size, p, len);
-	return p;
+	return memcpy(p, map->mmap + map->map_size, len);
 }