add protect for input number by user.

2014/08/05
Add protect conditions to judge input number.The fibonacci be solve by math formula.
So we know how to limit user to input wrong number into our program.
EX:pow(((1+(pow(5,1/2)))/2,n).  The (1+pow(5,1/2))/2 = 1.618.  pow(1.618,23)=64048.
Refer http://en.wikipedia.org/wiki/Fibonacci