Remove unneeded check for n == 0 in BigRational.reciprocal
Remove unneeded check for n == 0 in BigRational.reciprocal

0 will always be represented as LongRational(0,1), so BigRational will
never have n = 0. Also exchanged the comparison with the denominator with
a call to signum.

rklaehn/spire,AlecZorab/spire,woparry/spire,woparry/spire,codeaudit/spire,JackSullivan/spire,kevinmeredith/spire,siddhartha-gadgil/spire,guersam/spire,adampingel/spire,codeaudit/spire,siddhartha-gadgil/spire,kevinmeredith/spire,adampingel/spire,guersam/spire,tixxit/spire,JackSullivan/spire,non/spire,rklaehn/spire,tixxit/spire,non/spire,AlecZorab/spire