sqlite3: Fix _PyUnicode_AsStringAndSize() error handler.

Destroy begin_statement (not statement) on error.
diff --git a/Modules/_sqlite/connection.c b/Modules/_sqlite/connection.c
index 5738e21..2ce27f5 100644
--- a/Modules/_sqlite/connection.c
+++ b/Modules/_sqlite/connection.c
@@ -1087,7 +1087,7 @@
 
         statement = _PyUnicode_AsStringAndSize(begin_statement, &size);
         if (!statement) {
-            Py_DECREF(statement);
+            Py_DECREF(begin_statement);
             return -1;
         }
         self->begin_statement = PyMem_Malloc(size + 2);