B::walksymtable: clear cached methods

B::walksymtable: clear cached methods

There was a dummy assignment in B::walksymtable that I removed in com-
mit 6a4fc5265ba1 because it appeared to be redundant.  Removing that
assignment broke Module::Info (rt.cpan.org #123352), because it
changed the behaviour of B::Utils (by changing the behaviour of
B::walksymtable).  That seemingly useless assignment was actually
clearing cached methods, so that any B::GV object passed to the call-
back method sees ->CV pointing to something only if there is a real
sub there.  Since this seems like a reasonable expectation, this com-
mit restores the old behaviour, with a comment explaining what the
assignment is for, and tests it.