pmap segfaults with bad parameters?

I think we should sanitize our inputs, even if `.mapply` doesn't.
```r
> mlr3misc::pmap(1:4, function(x) x)

 *** caught segfault ***
address 0x200000001, cause 'memory not mapped'

Traceback:
 1: .mapply(.f, .x, list(...))
 2: mlr3misc::pmap(1:4, function(x) x)
```