palette/moreland: possible panic for Palette method

[Juneezee]

Minimal reproducible example:

package plotter_test

import (
	"testing"

	"gonum.org/v1/plot/palette/moreland"
)

func TestPalettePanic(t *testing.T) {
	minVal := 0.3402859786606234
	maxVal := 15.322841335211892
	n := 15

	t.Run("moreland.SmoothBlueRed panic", func(t *testing.T) {
		colors := moreland.SmoothBlueRed()
		colors.SetMin(minVal)
		colors.SetMax(maxVal)
		colors.Palette(n)
	})

	t.Run("moreland.Kindlmann panic", func(t *testing.T) {
		colors := moreland.Kindlmann()
		colors.SetMin(minVal)
		colors.SetMax(maxVal)
		colors.Palette(n)
	})
}

moreland.SmoothBlueRed:

=== RUN   TestPalettePanic
=== RUN   TestPalettePanic/moreland.SmoothBlueRed_panic
--- FAIL: TestPalettePanic/moreland.SmoothBlueRed_panic (0.00s)

--- FAIL: TestPalettePanic (0.00s)
panic: palette: specified value > maximum [recovered]
	panic: palette: specified value > maximum

goroutine 7 [running]:
testing.tRunner.func1.2({0x6d8d20, 0xe61110})
	/usr/local/go/src/testing/testing.go:1734 +0x21c
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1737 +0x35e
panic({0x6d8d20?, 0xe61110?})
	/usr/local/go/src/runtime/panic.go:792 +0x132
gonum.org/v1/plot/palette/moreland.smoothDiverging.Palette({{0x4054000000000000, 0x3ff147ae147ae148, 0xbff199999999999a}, {0x4054000000000000, 0x3ff147ae147ae148, 0x3fe0000000000000}, 0x4056000000000000, 0x3ff0000000000000, 0x3fd5c73ed7689f70, 0x402ea54b75a11069, ...}, ...)
	/home/jun/Desktop/github/waiting-for-pr/plot/palette/moreland/smooth.go:180 +0x1ab
gonum.org/v1/plot/plotter_test.TestPalettePanic.func1(0xc0002061c0?)
	/home/jun/Desktop/github/waiting-for-pr/plot/plotter/panic_test.go:18 +0x1b2
testing.tRunner(0xc0002061c0, 0xc00001e3e0)
	/usr/local/go/src/testing/testing.go:1792 +0xf4
created by testing.(*T).Run in goroutine 6
	/usr/local/go/src/testing/testing.go:1851 +0x413

moreland.Kindlmann:

=== RUN   TestPalettePanic
=== RUN   TestPalettePanic/moreland.Kindlmann_panic
--- FAIL: TestPalettePanic/moreland.Kindlmann_panic (0.00s)

--- FAIL: TestPalettePanic (0.00s)
panic: palette: specified value > maximum [recovered]
	panic: palette: specified value > maximum

goroutine 7 [running]:
testing.tRunner.func1.2({0x6d8d20, 0xe61110})
	/usr/local/go/src/testing/testing.go:1734 +0x21c
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1737 +0x35e
panic({0x6d8d20?, 0xe61110?})
	/usr/local/go/src/runtime/panic.go:792 +0x132
gonum.org/v1/plot/palette/moreland.luminance.Palette({{0xc000216000, 0xb, 0xb}, {0xc000218000, 0xb, 0xb}, 0x3ff0000000000000, 0x3fd5c73ed7689f70, 0x402ea54b75a11069}, 0xf)
	/home/jun/Desktop/github/waiting-for-pr/plot/palette/moreland/luminance.go:186 +0x177
gonum.org/v1/plot/plotter_test.TestPalettePanic.func2(0xc0002061c0?)
	/home/jun/Desktop/github/waiting-for-pr/plot/plotter/panic_test.go:25 +0x7f
testing.tRunner(0xc0002061c0, 0xc00001e4a0)
	/usr/local/go/src/testing/testing.go:1792 +0xf4
created by testing.(*T).Run in goroutine 6
	/usr/local/go/src/testing/testing.go:1851 +0x413