sf method for line_midpoint fails

As demonstrated below.

```r
library(stplanr)
l = osm_net_example
l$length_units = sf::st_length(l)
l$length_ = as.numeric(l$length_units)
hist(log(l$length))
l_points4 = line_midpoint(l_longest) # fails
plot(l_points4)
```