function stdev( p ) {
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	return sqrt( 1.0-p ) / p;
}
Returns the standard deviation of a geometric distribution.

@param {Probability} p - success probability
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 0.1 );
// returns ~9.487

@example
var v = stdev( 0.5 );
// returns ~1.414

@example
var v = stdev( 1.1 );
// returns NaN

@example
var v = stdev( NaN );
// returns NaN