message: Fix PluralFormsExpr::operator==() crash
  Fix crash when comparing two different expressions, at least one of
  them invalid. The bug was introduced in
diff: @@ -606,6 +606,9 @@ bool PluralFormsExpr::operator==(const PluralFormsExpr& other) const
  auto calc1 = calc();
  auto calc2 = other.calc();
  + if (!calc1 || !calc2)
  + return false; // at least one is invalid _and_ the strings are different due to code above
  +
  if (calc1->nplurals() != calc2->nplurals())
  return false;
  