fix(migration): Add column if not exists

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

core/Migrations/Version13000Date20170718121200.php

@@ -123,10 +123,12 @@ class Version13000Date20170718121200 extends SimpleMigrationStep {
 			$table->addIndex(['user_id', 'root_id', 'mount_point'], 'mounts_user_root_path_index', [], ['lengths' => [null, null, 128]]);
 		} else {
 			$table = $schema->getTable('mounts');
-			$table->addColumn('mount_id', Types::BIGINT, [
-				'notnull' => false,
-				'length' => 20,
-			]);
+			if (!$table->hasColumn('mount_id')) {
+				$table->addColumn('mount_id', Types::BIGINT, [
+					'notnull' => false,
+					'length' => 20,
+				]);
+			}
 			if (!$table->hasIndex('mounts_mount_id_index')) {
 				$table->addIndex(['mount_id'], 'mounts_mount_id_index');
 			}