use _safe_single_attr wrapper also for the map qualities

My merged VCF file ended up with an SQL error and I was able to track it down to the following variant.
This pull request should fix it.

```
'LOW', None, None, None, None, None, None, None, 8, None, 37.12, None, [0], 
4, None, None, '11.92', 2, None, None, None, None, False, 
None, None, None, False, True, '0.01', None, '0.07', None, '0.02', 
None, None, None, None, False, None, None, None, 'T', 'R', 
'R', 'R', 'R', 'T', None, None, 
<read-only buffer for 0x7f3431654318, size -1, offset 0 at 0x7f3431673970>, '-0.53', '1.59']
```