Guard against null pointers in actionBacktrace()

A warning was produced in GCC's LTO (GCC 13, Ubuntu 24.04) in
`-O2 -flto` mode:

```
In function ‘Process_isUserlandThread’,
    inlined from ‘actionBacktrace’ at Action.c:621:9:
Process.h:287:15: error: null pointer dereference [-Werror=null-dereference]
  287 |    return this->isUserlandThread;
      |               ^
```

It's good to add a null pointer guard anyway.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09

Action.c

@@ -618,10 +618,10 @@ static Htop_Reaction actionBacktrace(State *st) {
    const Vector* allProcesses = st->mainPanel->super.items;
 
    Vector* processes = Vector_new(Class(Process), false, VECTOR_DEFAULT_SIZE);
-   if (!Process_isUserlandThread(selectedProcess)) {
+   if (selectedProcess && !Process_isUserlandThread(selectedProcess)) {
       for (int i = 0; i < Vector_size(allProcesses); i++) {
          Process* process = (Process *)Vector_get(allProcesses, i);
-         if (Process_getThreadGroup(process) == Process_getThreadGroup(selectedProcess)) {
+         if (process && Process_getThreadGroup(process) == Process_getThreadGroup(selectedProcess)) {
             Vector_add(processes, process);
          }
       }