Fixed a bug with "pow" (#216)

contracts/src/libraries/FixedPointMath.sol

@@ -127,10 +127,19 @@ library FixedPointMath {
     /// @param y Fixed point number in 1e18 format.
     /// @return The result of x^y.
     function pow(uint256 x, uint256 y) internal pure returns (uint256) {
+        // If the exponent is 0, return 1.
+        if (y == 0) {
+            return ONE_18;
+        }
+
+        // If the base is 0, return 0.
+        if (x == 0) {
+            return 0;
+        }
+
         // Using properties of logarithms we calculate x^y:
         // -> ln(x^y) = y * ln(x)
         // -> e^(y * ln(x)) = x^y
-
         int256 y_int256 = int256(y); // solhint-disable-line var-name-mixedcase
 
         // Compute y*ln(x)

test/units/libraries/FixedPointMath.t.sol

@@ -152,10 +152,29 @@ contract FixedPointMathTest is Test {
     function test_pow() public {
         // NOTE: Coverage only works if I initialize the fixture in the test function
         MockFixedPointMath mockFixedPointMath = new MockFixedPointMath();
-        uint256 x = 300000000000000000000000;
-        uint256 y = 977464155968402951;
+
+        uint256 x = 0;
+        uint256 y = 0;
         uint256 result = mockFixedPointMath.pow(x, y);
         uint256 expected = LogExpMath.pow(x, y);
+        assertEq(result, expected);
+
+        x = 300000000000000000000000;
+        y = 0;
+        result = mockFixedPointMath.pow(x, y);
+        expected = LogExpMath.pow(x, y);
+        assertEq(result, expected);
+
+        x = 0;
+        y = 977464155968402951;
+        result = mockFixedPointMath.pow(x, y);
+        expected = LogExpMath.pow(x, y);
+        assertEq(result, expected);
+
+        x = 300000000000000000000000;
+        y = 977464155968402951;
+        result = mockFixedPointMath.pow(x, y);
+        expected = LogExpMath.pow(x, y);
         assertApproxEqAbs(result, expected, 1e5 wei);
 
         x = 180000000000000000000000;