Prevent nan when using color_to_hsv(white)

When r,g,b are equal, delta will be zero and hue will be nan due to
divide by zero.

This is also an issue with the reference C++ code:
https://tio.run/##Sy4o0E3OScxL//9fOTMvOac0JVXBJjO/uKQoNTHXjiszr0QhNzEzT0NToZqLs7gkxcoqOb@0RMHGRkHJIzUnJ19HITy/KCdFUQkkBJZPzUvJseaq/f8fAA

An alternative approach is to always add epsilon or use a different hue
calculation:
    http://web.archive.org/web/20210515082154/http://lolengine.net/blog/2013/01/13/fast-rgb-to-hsv

modules/color.lua

@@ -2,6 +2,7 @@
 -- @module color
 
 local modules  = (...):gsub('%.[^%.]+$', '') .. "."
+local constants = require(modules .. "constants")
 local utils    = require(modules .. "utils")
 local precond  = require(modules .. "_private_precond")
 local color    = {}
@@ -75,6 +76,11 @@ local function color_to_hsv(c)
 		return { h, s, v, 1 }
 	end
 
+	-- Prevent division by zero.
+	if delta == 0 then
+		delta = constants.DBL_EPSILON
+	end
+
 	if r == max then
 		h = ( g - b ) / delta     -- yellow/magenta
 	elseif g == max then