Fix segmentation fault from calling samtools depad on non-existant BAM

See also related issue #145, which has now been fixed but was causing an earlier segmentation fault. Before fix:

```
$ ./samtools depad does_not_exist.bam
[E::hts_open] fail to open file 'does_not_exist.bam'
[depad] failed to open does_not_exist.bam for reading.
Segmentation fault: 11
```

After this fix:

```
$ ./samtools depad does_not_exist.bam
[E::hts_open] fail to open file 'does_not_exist.bam'
[depad] failed to open does_not_exist.bam for reading.
```