Merge pull request #331 from nspcc-dev/vm-fix-things-for-96

This fixes two minor things observed in #96:

    running VM twice, which leads to instruction execution attempt for VM in FAULT state
    panicing with nil dereference (it's better to show some error message)

Before this patchset:

NEO-GO-VM > run
NEO-GO-VM > error encountered at instruction 6 (ROLL)
NEO-GO-VM > runtime error: invalid memory address or nil pointer dereference
FAULT
NEO-GO-VM > error encountered at instruction 7 (SETITEM)
NEO-GO-VM > interface conversion: interface {} is []vm.StackItem, not []uint8

After this patchset:

NEO-GO-VM > run
NEO-GO-VM > error encountered at instruction 6 (ROLL)
NEO-GO-VM > bad index
FAULT

pkg/vm/cli/cli.go

@@ -127,16 +127,13 @@ func (c *VMCLI) handleCommand(cmd string, args ...string) {
 		fmt.Printf("READY: loaded %d instructions\n", c.vm.Context().LenInstr())
 
 	case "run":
+		if len(args) != 0 {
 			var (
 				method []byte
 				params []vm.StackItem
 				err    error
 				start  int
 			)
-
-		if len(args) == 0 {
-			c.vm.Run()
-		} else {
 			if isMethodArg(args[0]) {
 				method = []byte(args[0])
 				start = 1
@@ -146,8 +143,8 @@ func (c *VMCLI) handleCommand(cmd string, args ...string) {
 				fmt.Println(err)
 				return
 			}
-		}
 			c.vm.LoadArgs(method, params)
+		}
 		c.vm.Run()
 
 	case "cont":

pkg/vm/vm.go

@@ -342,7 +342,11 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			panic("negative stack item returned")
 		}
 		if n > 0 {
-			v.estack.Push(v.estack.RemoveAt(n))
+			e := v.estack.RemoveAt(n)
+			if e == nil {
+				panic("bad index")
+			}
+			v.estack.Push(e)
 		}
 
 	case DROP: