Fix IF:<{ and IFNOT:<{ empty continuation case

[akifoq]

The boolean value should be DROPped in the case the continuation is empty.

To reproduce the bug, you can compare the following fift-assembler programs

"Asm.fif" include
<{ 42 PUSHINT IF:<{ }> }>s runvmcode .s // leaves 42 0

and

"Asm.fif" include
<{ 42 PUSHINT IF:<{ NOP }> }>s runvmcode .s // leaves 0

which obviously should be equivalent.

In some cases FunC compiler indeed produces empty IF:<{ branches, for example consider the following code

int main(int x) {
  int a = 1;
  if (x) { int a = 2; }
  return a;
}

which compiles into

main PROC:<{
  //  x
  1 PUSHINT	//  x a=1
  SWAP	//  a=1 x
  IF:<{	//  a=1
  }>	//  a=1
}>

(the bug was actually met in such situation)

[tvorogme]

@akifoq thank you for solution, it works for me.

Will patch toncli libs in next version.

[EmelyanenkoK]

Commits moved to https://github.com/newton-blockchain/ton/tree/akifoq-master
This PR will be open till merging akifoq-master