pp_readline: Don’t set PL_last_in_gv to &PL_sv_undef

Code elsewhere assumes it is always a GV or NULL:

readline "foo";
my $lastfh = "${^LAST_FH}";

pp_hot.c

@@ -449,6 +449,10 @@ PP(pp_readline)
 	    PUTBACK;
 	    Perl_pp_rv2gv(aTHX);
 	    PL_last_in_gv = MUTABLE_GV(*PL_stack_sp--);
+	    if (PL_last_in_gv == (GV *)&PL_sv_undef)
+		PL_last_in_gv = NULL;
+	    else
+		assert(isGV_with_GP(PL_last_in_gv));
 	}
     }
     return do_readline();

t/op/readline.t

@@ -6,7 +6,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 
-plan tests => 30;
+plan tests => 31;
 
 # [perl #19566]: sv_gets writes directly to its argument via
 # TARG. Test that we respect SvREADONLY.
@@ -270,6 +270,11 @@ is tell, -1, 'unglobbery of last gv nullifies PL_last_in_gv';
 readline *{$f{g}};
 is tell, tell *foom, 'readline *$glob_copy sets PL_last_in_gv';
 
+# PL_last_in_gv should not point to &PL_sv_undef, either.
+# This used to fail an assertion or return a scalar ref.
+readline undef;
+is ${^LAST_FH}, undef, '${^LAST_FH} after readline undef';
+
 __DATA__
 moo
 moo