Bug in parallel exitWith branch if a branch return undefined

The bug appears when the branch returning undefined finished first.
For example:
```
parallel exitWith branch 1 ||
awaitClickBeep('body')
||
js ():
return
```
gives the error message:
```
multiple value for cancellor variable --IN-- "... {foreach @all __par
.var result
:set result $__par
{if [.get __par_N = 0]
[ num set $result ]
else
:set retValMul $result
{if [.get __par_N = $num]
:breakReturn all $retValMul
}
}
}
```
which is not informative.