Avoid signed overflow in moc

moc's preprocessor needs to implement certain math operation to
correctly handle #if conditions. Unfortunately, its implementation is
not overflow safe. However, those are rare enough in practice that we
in general do not need to care about them.
This patch adds a workaround for one case where UBSAN run into an
overflow related issue.
A complete fix would require to make moc spec compliant (do math with
std::max_(u)int_t operands; always wrap on overflow) in all operations.

Pick-to: 6.0 5.15
Fixes: QTBUG-88825
Change-Id: Ic4d2cb097db2fa2f9d4681bbaab3068eaa2745aa
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/tools/moc/preprocessor.cpp

@@ -886,7 +886,15 @@ int PP_Expression::multiplicative_expression()
     int value = unary_expression();
     switch (next()) {
     case PP_STAR:
-        return value * multiplicative_expression();
+    {
+        // get well behaved overflow behavior by converting to long
+        // and then back to int
+        // NOTE: A conformant preprocessor would need to work intmax_t/
+        // uintmax_t according to [cpp.cond], 19.1 §10
+        // But we're not compliant anyway
+        qint64 result = qint64(value) * qint64(multiplicative_expression());
+        return int(result);
+    }
     case PP_PERCENT:
     {
         int remainder = multiplicative_expression();