storage/State: check if a CompositeStorage exists; fixes nullptr dereference

Fixes another crash bug caused by commit
64d141f71e

NEWS

@@ -1,4 +1,7 @@
 ver 0.20.15 (not yet released)
+* state file
+  - make mount point restore errors non-fatal
+  - fix crash when restoring mounts with incompatible database plugin
 
 ver 0.20.14 (2018/01/01)
 * database

src/storage/StorageState.cxx

@@ -48,6 +48,9 @@ static constexpr Domain storage_domain("storage");
 void
 storage_state_save(BufferedOutputStream &os, const Instance &instance)
 {
+	if (instance.storage == nullptr)
+		return;
+
 	const auto visitor = [&os](const char *mount_uri, const Storage &storage) {
 		std::string uri = storage.MapUTF8("");
 		if (uri.empty() || StringIsEmpty(mount_uri))
@@ -85,6 +88,12 @@ storage_state_restore(const char *line, TextFile &file, Instance &instance)
 			FormatError(storage_domain, "Unrecognized line in mountpoint state: %s", line);
 	}
 
+	if (instance.storage == nullptr)
+		/* without storage (a CompositeStorage instance), we
+		   cannot mount, and therefore we silently ignore the
+		   state file */
+		return true;
+
 	if (url.empty() || uri.empty()) {
 		LogError(storage_domain, "Missing value in mountpoint state.");	
 		return true;
@@ -120,6 +129,9 @@ storage_state_restore(const char *line, TextFile &file, Instance &instance)
 unsigned
 storage_state_get_hash(const Instance &instance)
 {
+	if (instance.storage == nullptr)
+		return 0;
+
 	std::set<std::string> mounts;
 
 	const auto visitor = [&mounts](const char *mount_uri, const Storage &storage) {