Remove some unreasonable input values for rootn

Remove some unreasonable input values for rootn

In manual for function pow(), there's following description:
	"If x is a finite value less than 0,
	and y is a finite noninteger,
	a domain error occurs, and a NaN is returned."
That means we can't calculate rootn in cpu like this pow(x,1.0/y) which is mentioned in OpenCL spec.
E.g. when y=3 and x=-8, rootn should return -2. But when we calculate pow(x, 1.0/y), it will return a Nan.
I didn't find multi-root math function in glibc.

Signed-off-by: Yi Sun <yi.sun@intel.com>