JIT match failure for empty capture group

Discovered by #322.

This seems to be an outright incorrect behaviour in the JIT, and it's not an extreme edge case.

```
  re> /(1|)b*1/
data> (1|)b*1       
No match
data> (1|)b*1\=no_jit
 0: 1
 1: 
```

Note that the following cases operate as expected:
```
  re> /1?a*1/
data> 1
 0: 1
data> 
  re> /(1?)a*1/
data> 1
 0: 1
 1: 
data> 1\=no_jit
 0: 1
 1:
```