ProFunctionDef is move-enabled, meaning its `m_pro` field can
become nullptr. Its usage in the assignment operator and the dtor
must therefore be protected with a check.
Amends 9c63ad562b.
Change-Id: I0c77b07dc83969565480bbb9d9fc80751d4246b1
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>