Fix assertion failure with -DT -e\"

$ ./perl -DT -e\"
Assertion failed: (s), function S_printbuf, file toke.c, line 479.
Abort trap: 6
Don’t let reblessing point SvSTASH to a half-freed stash

We should set SvSTASH before calling SvREFCNT_dec on its old value,
otherwise we could free the old stash and trigger a DESTROY that sees
the object blessed into the half-freed stash.

Currently, a second call to bless inside DESTROY can trigger ‘Attempt
to free unreferenced scalar’, because it calls SvREFCNT_dec a second
time on the stash that already has a reference count of 0.
Fix -DP output

broken by b66f3475d343.

$ ./perl -Ilib -DP
Screamer: entering, ptr=140185618803200, cnt=0
Screamer: entering: PerlIO * thinks ptr=140185618803200, cnt=%z, base=0
Screamer: going to getc, ptr=140185618803200, cnt=%z
^C

The format is %zd, not %z.
Fix buffer overflow with overlong identifiers

This was introduced by commit 32833930e32dc in 5.17.10.

$  ./perl -Ilib -e Foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
Identifier too long at -e line 1.
Segmentation fault: 11

(That was an amusing use of macro parentheses for the while condition,
at least while it lasted.)
perlio.c: misuse of SvPV_nolen_const

SvPV_nolen_const takes only one argument and does not return
a length.