improve mum performance and reduce gas cost

contracts/math/SafeMath.sol

@@ -7,8 +7,11 @@ pragma solidity ^0.4.11;
  */
 library SafeMath {
   function mul(uint256 a, uint256 b) internal constant returns (uint256) {
+    if (a == 0) {
+      return 0;
+    }
     uint256 c = a * b;
-    assert(a == 0 || c / a == b);
+    assert(c / a == b);
     return c;
   }