[CRIU] Fix bugs in the missing pid code

[Tycho Andersen]

Hi Pavel, Andrey,

Here are patches that fix the two bugs Pavel pointed out in the remap dead pids
code. However the test doesn't pass, it seems that something in the series
starting at e46a9f6bfc65a2ce1996972a00df6e14cb64f517 causes it to hang. I am
looking into this now, but here are these in the meantime if you want to apply
them (I tested them via cherry picking and they seem ok).

Tycho