Fixing null pointer dereference.

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index b9f45ec480c8af44e03309b060abd07360582a1c..2722ffdc7f9d51557090b2300d1514135b9737bd 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -370,8 +370,9 @@ class SQL_EXPORT Connection {
     // When true, the statement can be used.
     bool is_valid() const { return !!stmt_; }
 
-    // If we've not been linked to a connection, this will be NULL. Guaranteed
-    // non-NULL when is_valid().
+    // If we've not been linked to a connection, this will be NULL.
+    // TODO(shess): connection_ can be NULL in case of GetUntrackedStatement(),
+    // which prevents Statement::OnError() from forwarding errors.
     Connection* connection() const { return connection_; }
 
     // Returns the sqlite statement if any. If the statement is not active,