Shell: Do not assume that wstatus is valid after wait() returns 0

According to the linux waitid manpage, the value of wstatus is
unspecified if wait() returns 0, so we should not assume that any
value it holds is correct (including the exit code).
This is only applicable to the Lagom build.

Shell/main.cpp

@@ -99,7 +99,7 @@ int main(int argc, char** argv)
 #ifndef __serenity__
  if (child_pid == 0) {
  // Linux: if child didn't "change state", but existed.
- child_pid = job.value->pid();
+ continue;
  }
 #endif
  if (child_pid == job.value->pid()) {