Error in if (is.nan(p0) || p0 < a2 || p0 > (1 - a2)) {: missing value where TRUE/FALSE needed

Equivalent of https://github.com/bfast2/strucchange/issues/18 in bfast01, with a trivial reproducer:

``` r
library(bfast)
bfast01(data = Nile)
#> Error in if (is.nan(p0) || p0 < a2 || p0 > (1 - a2)) {: missing value where TRUE/FALSE needed
```

^{Created on 2020-03-27 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)}

Blame: https://github.com/bfast2/bfast/blob/f58ba97a3dc2b98ea06f265a10f81c19c823c323/R/bfast01.R#L462

But really should just depend on strucchange to not duplicate code.

Start with the reproducer in the issue and inspect R/bfast01.R around line 462, where the failing condition is identified. Compare the duplicated logic with the linked strucchange issue and determine how bfast should rely on strucchange instead. Done means bfast01(data = Nile) no longer raises the missing-value error and the relevant behavior remains covered.

Written by the indexing model from the issue text.