core, tests: reduced state copy by N calls

Reduced the amount of state copied that are required by N calls by doing
a balance check prior to any state modifications.

core/vm/instructions.go

@@ -13,6 +13,7 @@
 //
 // You should have received a copy of the GNU Lesser General Public License
 // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
 package vm
 
 import (