sv.c: Don’t flip flags so many times in glob_assign_glob

sv.c: Don’t flip flags so many times in glob_assign_glob

We only need to turn off the SvOK flags when the destination was not
already a glob.

Up till now, the code was doing SvOK_off twice, and
isGV_with_GP_on/off several times to avoid assertion failures.

If we already have a glob, then the only SvOK flags that can be on
already are SVpgv_GP (which needs to be on) and possibly SVf_UTF8,
which is harmless.

The only difference is that now the dumped glob in this one-liner will
have its UTF8 flag set:

$ ./perl -Ilib -MDevel::Peek -e 'use utf8; "".*φου; *φου = *bar; Dump *φου'

whereas it didn’t before.  That is of no consequence, as the UTF8 flag
is only meaningful after stringification, which resets it.