Fixed recursive stack overflow when setting operator. If division by …

…0 was attempted, and the operator was then changed, the app would end up in a recursive loop.

app/src/main/java/com/akseltorgard/devcalc/Calculator.java

@@ -276,7 +276,7 @@ void setOperator(Operators.BinaryOperator operator) {
         }
 
         //Operator pressed before any digit has been input
-        else if (mInput == null) {
+        else if (mInput == null || mInput == 0) {
             mOperator = operator;
         }