fix nonsense output from k-rate phasorbnk

The phasorbnk opcode is supposed to return a value in the range 0 to 1.  When used at a-rate, that is accomplished in a fairly obvious way:  if the value we are about to return is greater than 1, subtract 1, and if it's less than 0, add 1.  (It is assumed that it will always be between -1 and 2 going into this check.)  But when used at k-rate, the second check tests against 1, not 0.  Thus, if the value was in range going into the check, it is guaranteed _not_ to be in range afterward.  This patch fixes that.  It looks like I may be the first person to attempt serious use of phasorbnk at k-rate in the 15 years since it was added to Csound, since this bug makes phasorbnk effectively unusable at k-rate.