QSqlError: protect against self-assignment

Address static analyzer warning 6eb3060fd64e459b6cd5586dc561e0ba.

Also make assignment from a moved-from object safe.

Task-number: QTBUG-91912
Change-Id: I732dc244ac0c731a02d85e88023dbd952b9eb112
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/sql/kernel/qsqlerror.cpp

@@ -151,9 +151,13 @@ QSqlError::QSqlError(const QSqlError& other)
 
 QSqlError& QSqlError::operator=(const QSqlError& other)
 {
-    if (d)
+    if (&other == this)
+        return *this;
+    if (d && other.d)
         *d = *other.d;
-    else
+    else if (d)
+        *d = QSqlErrorPrivate();
+    else if (other.d)
         d = new QSqlErrorPrivate(*other.d);
     return *this;
 }