Fix assertion failure with qr/\Q(?{})/

\Q and \u  create ops that need targets, and hence use the pad of
the anonymous sub created temporarily when parsing something like
qr/\Q(?{})/.  If it turns out we don’t have a code block (in this
case), that anon sub is thrown away, but there is an assertion that
makes sure its pad has not been used, which fails:

$ ./perl -e 'qr/\Q(?{})/'
Assertion failed: (AvFILLp(PL_comppad) == 0), function Perl_pmruntime, file op.c, line 5395.
Abort trap: 6

(That assertion was added by d63c20f27.)

If we have had \Q or \l, then the length of the pad may be more than
1, but constant folding should have stolen the values from the pad, so
assert that instead.