CutN fails when sequence begins with Ns

Using seqtk version 1.4-r122 seqt fails to identify Ns at the beginning of the contig 

Example 
```
>test
NNNNNNNATCTA
>test2
AATTCNNNA

seqtk cutN -g -n1 test.fa
test2	5	8

seqtk gap -l 1 test.fa
test	0	7
test2	5	8
```

CutN identifies the stretch of Ns in the second contig but not if the first which begins with Ns. Seqtk gap works as expected. 