db/simple: fix implicit nullptr/bool conversion

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -448,13 +448,13 @@ SimpleDatabase::Mount(const char *uri, Database *db, Error &error)
 	if (r.uri == nullptr) {
 		error.Format(db_domain, DB_CONFLICT,
 			     "Already exists: %s", uri);
-		return nullptr;
+		return false;
 	}
 
 	if (strchr(r.uri, '/') != nullptr) {
 		error.Format(db_domain, DB_NOT_FOUND,
 			     "Parent not found: %s", uri);
-		return nullptr;
+		return false;
 	}
 
 	Directory *mnt = r.directory->CreateChild(r.uri);
@@ -481,7 +481,7 @@ SimpleDatabase::Mount(const char *local_uri, const char *storage_uri,
 	if (cache_path.IsNull()) {
 		error.Format(db_domain, DB_NOT_FOUND,
 			     "No 'cache_directory' configured");
-		return nullptr;
+		return false;
 	}
 
 	std::string name(storage_uri);

src/pcm/FormatConverter.cxx

@@ -44,7 +44,7 @@ PcmFormatConverter::Open(SampleFormat _src_format, SampleFormat _dest_format,
 			     "PCM conversion from %s to %s is not implemented",
 			     sample_format_to_string(_src_format),
 			     sample_format_to_string(_dest_format));
-		return nullptr;
+		return false;
 
 	case SampleFormat::S16:
 	case SampleFormat::S24_P32:

src/storage/plugins/NfsStorage.cxx

@@ -288,7 +288,7 @@ NfsStorage::GetInfo(const char *uri_utf8, gcc_unused bool follow,
 		return false;
 
 	if (!WaitConnected(error))
-		return nullptr;
+		return false;
 
 	NfsGetInfoOperation operation(*connection, path.c_str(), info);
 	return operation.Run(error);