Kernel: Add child to scoped process list after last TRY() in posix_spawn

Before this point, creating the child can still fail, so we shouldn't
register it yet. Adding the child to the scoped process list now
happens immediately before the process is registered by
Process::register_new().

Kernel/Syscalls/posix_spawn.cpp

@@ -100,6 +100,11 @@ ErrorOr<FlatPtr> Process::sys$posix_spawn(Userspace<Syscall::SC_posix_spawn_para
     // A child created via posix_spawn inherits a copy of its parent's signal mask
     child_first_thread->update_signal_mask(Thread::current()->signal_mask());
 
+    Thread* new_main_thread = nullptr;
+    InterruptsState previous_interrupts_state = InterruptsState::Enabled;
+    TRY(child->exec(move(path), move(arguments), move(environment), new_main_thread, previous_interrupts_state));
+    thread_finalizer_guard.disarm();
+
     m_scoped_process_list.with([&](auto const& list_ptr) {
         if (list_ptr) {
             child->m_scoped_process_list.with([&](auto& child_list_ptr) {
@@ -109,11 +114,6 @@ ErrorOr<FlatPtr> Process::sys$posix_spawn(Userspace<Syscall::SC_posix_spawn_para
         }
     });
 
-    Thread* new_main_thread = nullptr;
-    InterruptsState previous_interrupts_state = InterruptsState::Enabled;
-    TRY(child->exec(move(path), move(arguments), move(environment), new_main_thread, previous_interrupts_state));
-    thread_finalizer_guard.disarm();
-
     Process::register_new(*child);
 
     // NOTE: All user processes have a leaked ref on them. It's balanced by Thread::WaitBlockerSet::finalize().