math: no check for negative precisions which result in runtime panics when passed into *NewDec

[odeke-em]

Noticed in a security audit that this code for LegacyNewDec only checks that precision > LegacyPrecision

cosmos-sdk/math/dec.go

Lines 81 to 86 in 00ad3ec

	func precisionMultiplier(prec int64) *big.Int {
	if prec > LegacyPrecision {
	panic(fmt.Sprintf("too much precision, maximum %v, provided %v", LegacyPrecision, prec))
	}
	return precisionMultipliers[prec]
	}

but then blindly passes the value to index precisionMultipliers

Reproduction:

func TestDecPrecision(t *testing.T) {
        LegacyNewDecWithPrec(10, -1)
}

which panics with

panic: runtime error: index out of range [-1] [recovered]
	panic: runtime error: index out of range [-1]

goroutine 19 [running]:
testing.tRunner.func1.2({0x1475040, 0xc000136288})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/testing/testing.go:1506 +0x24e
testing.tRunner.func1()
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/testing/testing.go:1509 +0x39f
panic({0x1475040, 0xc000136288})
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/runtime/panic.go:884 +0x213
cosmossdk.io/math.precisionMultiplier(...)
	/Users/emmanuelodeke/go/src/github.com/cosmos/cosmos-sdk/math/dec.go:85
cosmossdk.io/math.LegacyNewDecWithPrec(0xa, 0xffffffffffffffff)
	/Users/emmanuelodeke/go/src/github.com/cosmos/cosmos-sdk/math/dec.go:97 +0xd9
cosmossdk.io/math.TestDecPrecision(0x0?)
	/Users/emmanuelodeke/go/src/github.com/cosmos/cosmos-sdk/math/panic_test.go:27 +0x25
testing.tRunner(0xc000103860, 0x14d2748)
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/testing/testing.go:1556 +0x10b
created by testing.(*T).Run
	/Users/emmanuelodeke/go/src/go.googlesource.com/go/src/testing/testing.go:1603 +0x35f
exit status 2

Even though the validation is to panic, at least let's ensure it is intended as opposed to being unintended.