multiple substitution doesn't fail when too many substitutions are specified

failure in S05-modifier/counted.t

There are only six possible substitutions here, but the 7x doesn't fail as expected:

```
niecza> my $data = "f fo foo fooo foooo fooooo foooooo";
f fo foo fooo foooo fooooo foooooo
niecza> $data ~~ s:7x{fo+}=q{bar}, 'substitute 7x'
True substitute 7x
```
