Make rv2gv return autovivified magic GVs

There is special code in pp_rv2gv to deal with the case of built-in
variables that are created on the fly.  It basically pretends that
they have always existed, even in rvalue context.

Normally, defined(*{"foo"}) will not actually create the *foo glob,
but will simply return false.  defined(*{">"}), however is supposed
to return true because of the $> variable; its popping into existing
when looked at being an implementation detail.  That is the whole pur-
pose of is_gv_magical_sv in gv.c.

Prior to this commit, however, defined(*{">"}) would autovivify the
GV, but then return *false*!

It was simply a matter of faulty logic in this part of pp_rv2gv:

		SV * const temp = MUTABLE_SV(gv_fetchsv(sv, 0, SVt_PVGV));
		if (!temp
		    && (!is_gv_magical_sv(sv,0)
			|| !(sv = MUTABLE_SV(gv_fetchsv(sv, GV_ADD,
							SVt_PVGV))))) {
		    RETSETUNDEF;
		}
		sv = temp;

The autovivification happens in the second gv_fetchsv call.  But after
the new GV is assigned to sv and the condition proves false, we reach
the sv = temp assignment which clobbers it.

Author: Father Chrysostomos

pp.c

@@ -207,7 +207,7 @@ PP(pp_rv2gv)
 							SVt_PVGV))))) {
 		    RETSETUNDEF;
 		}
-		sv = temp;
+		if (temp) sv = temp;
 	    }
 	    else {
 		if (PL_op->op_private & HINT_STRICT_REFS)

t/op/readdir.t

@@ -59,8 +59,12 @@ isnt("$fh", "$fh{abc}");
 
 # See that perl does not segfault upon readdir($x="."); 
 # http://rt.perl.org/rt3/Ticket/Display.html?id=68182
+# This test has subsequently changed to use "x", because *. now always
+# returns a glob (because of the $. variable), even in the non-vivifying
+# context supplied by readdir (which implies *{}). "x" triggers the same
+# conditions under which the segfault originally occurred.
 fresh_perl_like(<<'EOP', qr/^Bad symbol for dirhandle at -/, {}, 'RT #68182');
-    my $x = ".";
+    my $x = "x";
     my @files = readdir($x);
 EOP