Demonstrate panic

[sethvargo]

go test ./language
--- FAIL: TestRegionGroups (0.00s)
panic: runtime error: index out of range [357] with length 357 [recovered]
	panic: runtime error: index out of range [357] with length 357

goroutine 357 [running]:
testing.tRunner.func1.1(0x138cec0, 0xc000494c60)
	/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1072 +0x30d
testing.tRunner.func1(0xc0004c7380)
	/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1075 +0x41a
panic(0x138cec0, 0xc000494c60)
	/usr/local/Cellar/go/1.15.6/libexec/src/runtime/panic.go:969 +0x1b9
golang.org/x/text/language.regionGroupDist(0x139005a00cf0165, 0x141b400)
	/Users/sethvargo/Development/text/language/match.go:676 +0x13b
golang.org/x/text/language.TestRegionGroups(0xc0004c7380)
	/Users/sethvargo/Development/text/language/match_test.go:178 +0x467
testing.tRunner(0xc0004c7380, 0x13cac60)
	/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1123 +0xef
created by testing.(*T).Run
	/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1168 +0x2b3
FAIL	golang.org/x/text/language	0.138s
FAIL

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[sethvargo]

@seankhliao please see this reproduction example