signal_impl::notify(): Don't delete signal_impl during erase()
signal_impl::notify(): Don't delete signal_impl during erase()

* sigc++/signal_base.cc: signal_impl::notify() and sweep(): Don't delete
signal_impl while its slot list is being manipulated. Without this fix,
tests/test_bind_refptr crashes in MS Visual C++ in debug mode.
The behaviour without the fix is risky, and shall be avoided. Bug #564005.

Distrotech/libsigcxx2,Distrotech/libsigcxx2,Distrotech/libsigcxx2