Prevent failure of binning algorithm in rare cases.

[clauswilke]

This closes #3606:

library(ggplot2)
ggplot() + geom_histogram(aes(x= rep(0.25,10)))
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2019-11-05 by the reprex package (v0.3.0)}

[yutannihilation]

Looks good.

[lock]

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