Segmentation fault on empty files

`bioawk` segfaults when asked to parse an empty files

```bash
$ touch test.fastq
$ gzip test.fastq 
$ bioawk -c fastx '{print}' test.fastq.gz 
Segmentation fault
```

Actually, it also segfaults on non-gzipped input:

```bash
$ touch test.fastq
$ bioawk -c fastx '{print}' test.fastq
Segmentation fault
```

Is it something easily fixable?