Call get-magic once for defined ${...}

Call get-magic once for defined ${...}

This example:

sub TIESCALAR { bless[]}
sub FETCH { warn "fetching"; "\cTAINT" }
tie my $a, "";
defined $$a;

prints ‘fetching’ three times in 5.8.8, five times (!) in 5.10-12,
four times in 5.14, and three times in blead as of ed996e63f6.  Now it
only happens once.

It was commit 7a5fd60d4c that increased the number of fetches in 5.10,
but I haven’t checked which commits reduced it in 5.14.