lib: add extra nullptr check to get_freebusy

```
(gdb) bt
f0  TPROPVAL_ARRAY::find (tag=0x80030102, this=0x0) at include/gromox/mapidefs.h:1198
f1  TPROPVAL_ARRAY::getval (this=0x0, tag=0x80030102) at include/gromox/mapidefs.h:1211
f2  TPROPVAL_ARRAY::get<BINARY> (tag=<optimized out>, this=<optimized out>) at include/gromox/mapidefs.h:1219
f3  get_freebusy (username=username@entry=0x7f8610410f34 "[email protected]", dir=<optimized out>,
    dir@entry=0x7f85e0a8b790 "/var/lib/gromox/user/A/1", start_time=start_time@entry=1758852000, end_time=end_time@entry=1761271200,
    fb_data=std::vector of length 210, capacity 256 = {...}) at lib/freebusy.cpp:418
418                     if (!goid_to_icaluid(ctnt->proplist.get<BINARY>(ptag.globalobjectid), uid_buf))
(gdb) p ctnt
$2 = (message_content *) 0x0
```

This should not be happening; might be caused by a different bug.

References: GXL-503, DESK-3987

Author: jengelh

lib/freebusy.cpp

@@ -418,9 +418,9 @@ bool get_freebusy(const char *username, const char *dir, time_t start_time,
 	for (size_t i = 0; i < rows.count; ++i) {
 		message_content *ctnt = nullptr;
 		auto msgid = rows.pparray[i]->get<const uint64_t>(PidTagMid);
-		if (msgid == nullptr)
-			continue;
-		if (!exmdb_client->read_message(dir, nullptr, CP_ACP, *msgid, &ctnt))
+		if (msgid == nullptr ||
+		    !exmdb_client->read_message(dir, nullptr, CP_ACP, *msgid, &ctnt) ||
+		    ctnt == nullptr)
 			continue;
 		std::string uid_buf;
 		if (!goid_to_icaluid(ctnt->proplist.get<BINARY>(ptag.globalobjectid), uid_buf))