LibSQL+SQLServer: Do not re-open databases

Both `Database` and `Heap` were allowed to be opened twice. Prevent
this, and change SQLServer to only open databases that are not already
opened.

This fixes a Ladybird crash where opening the application twice would
erroneously duplicate free heap block indices.

Userland/Libraries/LibSQL/Database.cpp

@@ -23,6 +23,7 @@ Database::Database(DeprecatedString name)
 
 ResultOr<void> Database::open()
 {
+    VERIFY(!m_open);
     TRY(m_heap->open());
 
     m_schemas = BTree::construct(m_serializer, SchemaDef::index_def()->to_tuple_descriptor(), m_heap->schemas_root());

Userland/Libraries/LibSQL/Heap.cpp

@@ -29,6 +29,8 @@ Heap::~Heap()
 
 ErrorOr<void> Heap::open()
 {
+    VERIFY(!m_file);
+
     size_t file_size = 0;
     struct stat stat_buffer;
     if (stat(name().characters(), &stat_buffer) != 0) {

Userland/Services/SQLServer/DatabaseConnection.cpp

@@ -38,11 +38,12 @@ ErrorOr<NonnullRefPtr<DatabaseConnection>> DatabaseConnection::create(StringView
         return Error::from_string_view("Invalid database name"sv);
 
     auto database = TRY(find_or_create_database(database_path, database_name));
-
+    if (!database->is_open()) {
         if (auto result = database->open(); result.is_error()) {
             warnln("Could not open database: {}", result.error().error_string());
             return Error::from_string_view("Could not open database"sv);
         }
+    }
 
     return adopt_nonnull_ref_or_enomem(new (nothrow) DatabaseConnection(move(database), move(database_name), client_id));
 }