fix: null array indexing in CTracer_handle_return

CTracer_set_pdata_stack can initialize self.pdata_stack to an empty
stack where ->stack is NULL and ->deph is -1.

Move index into ->stack into ->depth>=0 check to avoid indexing into
NULL array.

This issue was found running UndefinedBehaviourSanitizer. It's
reproducible in regular test runs, e.g. `python3 -m tox -e py311`.
Adding `if (self->pdata_stack->stack == NULL) { fprint(...) }` before
the moved line shows the issue in several test cases.

coverage/ctracer/tracer.c

@@ -722,9 +722,9 @@ CTracer_handle_return(CTracer *self, PyFrameObject *frame)
     if (CTracer_set_pdata_stack(self) < 0) {
         goto error;
     }
-    self->pcur_entry = &self->pdata_stack->stack[self->pdata_stack->depth];
 
     if (self->pdata_stack->depth >= 0) {
+        self->pcur_entry = &self->pdata_stack->stack[self->pdata_stack->depth];
         if (self->tracing_arcs && self->pcur_entry->file_data) {
             BOOL real_return = FALSE;
             pCode = MyCode_GetCode(MyFrame_GetCode(frame));