In-place edit (`-i`) issues (was: assertion failure in main.c line 518 & coredump)

This behaviour seems a little severe:

```
$ jq --version
jq-1.5rc1-31-g3b8d081
$ cat a
[1,2,3]
$ cat b
{"a":4,"b":5,"c":6}
$ jq '.' -i a b
Assertion failed: first_file != 0 && !strcmp(first_file, "-"), file main.c, line 518, function main
Abort (core dumped)
```
