Avoid the risk of leaking small amounts of memory.

Removing from the bottom of a stack (nipN(depth)) would leak the first
entry in the array by slicing [1:], leaving array[0] dangling an
inaccessible (but unable to be freed until the whole slice is gone). We
left it like this for a while, but best not to leak the memory. Happens
rarely so the performance hit shouldn't matter that much. Do the same
thing for condstack.

opcode.go

@@ -840,7 +840,9 @@ func opcodeEndif(op *parsedOpcode, s *Script) error {
 		return StackErrNoIf
 	}
 
-	s.condStack = s.condStack[1:]
+	stk := make([]int, len(s.condStack) -1, len(s.condStack) -1)
+	copy(stk, s.condStack[1:])
+	s.condStack = stk
 	return nil
 }
 

stack.go

@@ -173,7 +173,9 @@ func (s *Stack) nipN(idx int) (so []byte, err error) {
 	if idx == 0 {
 		s.stk = s.stk[:sz-1]
 	} else if idx == sz-1 {
-		s.stk = s.stk[1:]
+		s1 := make([][]byte, sz - 1, sz - 1)
+		copy(s1, s.stk[1:])
+		s.stk = s1
 	} else {
 		s1 := s.stk[sz-idx : sz]
 		s.stk = s.stk[:sz-idx-1]