Warn too many breaks before doing seq()

[yutannihilation]

Fix #4131

devtools::load_all("~/repo/ggplot2")
#> Loading ggplot2

ggplot(tibble::tibble(x=c(1, 1e12)), aes(x)) + geom_histogram(binwidth=1)
#> Warning: Computation failed in `stat_bin()`:
#> The number of histogram bins must be less than 1,000,000.
#> Did you make `binwidth` too small?

^{Created on 2020-07-11 by the reprex package (v0.3.0)}