[perl #97988] Nullify PL_last_in_gv when unglobbed

[perl #97988] Nullify PL_last_in_gv when unglobbed

Code like this can cause PL_last_in_gv to point to a coercible glob:

    $f{g} = *STDOUT;
    readline $f{g};

If $f{g} is then modified such that it is no longer a glob,
PL_last_in_gv ends up pointing to a non-glob:

    $f{g} = 3;

If $f{g} is freed now, the PL_last_in_gv-nulling code in sv_clear will
be skipped, as it only applies to globs.

    undef %f; # now PL_last_in_gv points to a freed scalar

The resulting freed scalar can be reused by another handle,

    *{"foom"} = *other;

causing tell() with no arguments to return the position on *other,
even though *other was no the last handle read from.

This commit fixes it by nulling PL_last_in_gv when a coercible glob
is coerced.