Move check for NULL boss into call_boss()

Negligible perf impact and makes it safe to use in different contexts

kitty/screen.c

@@ -1772,7 +1772,7 @@ screen_update_selection(Screen *self, index_type x, index_type y, bool ended) {
             break;
     }
     if (found) self->selection.end_x = end;
-    if (global_state.boss) call_boss(set_primary_selection, NULL);
+    call_boss(set_primary_selection, NULL);
 }
 
 static PyObject*

kitty/state.h

@@ -137,7 +137,7 @@ typedef struct {
 
 extern GlobalState global_state;
 
-#define call_boss(name, ...) { \
+#define call_boss(name, ...) if (global_state.boss) { \
     PyObject *cret_ = PyObject_CallMethod(global_state.boss, #name, __VA_ARGS__); \
     if (cret_ == NULL) { PyErr_Print(); } \
     else Py_DECREF(cret_); \