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) {