PGPRO-11098 add null-pointer check to ptrack_set_init_lsn() function

When ptrack extension is added to shared_preload_libraries, but
ptrack.map size is 0, we may encounter segmantation fault when calling
ptrack_backup_checkpoint_request_hook() function. This commit adds
additional null-pointer checks to ptrack_set_init_lsn() function called
by ptrack_backup_checkpoint_request_hook() to avoid this segfault

engine.c

@@ -659,6 +659,10 @@ XLogRecPtr
 ptrack_set_init_lsn(void)
 {
 	XLogRecPtr new_lsn;
+
+	if (ptrack_map_size == 0 || ptrack_map == NULL)
+		return InvalidXLogRecPtr;
+
 	if (RecoveryInProgress())
 		new_lsn = GetXLogReplayRecPtr(NULL);
 	else