19196: cpu/esp32: fixes for boot issues and crashes on ESP32 [backport 2023.1] r=benpicco a=gschorcht

# Backport of PR #19192

### Contribution description

In `syscalls_init()` there is a call to `malloc()`, which will return `NULL` if the heap is not initialized before, causing the entire board to fail booting if module `esp_idf_heap` is used, for example if the WiFi network interface is used.

API of [vTaskDelete()](https://www.freertos.org/a00126.html) says, that if NULL is passed to vTaskDelete the calling task should be deleted.

This PR needs a backport to 2023.01.

### Testing procedure

```
USEMODULE=esp_idf_heap BOARD=esp32-wroom-32 make -C tests/malloc
````
should work now but hangs in startup without the PR.

### Issues/PRs references

Issue was introduced with PR #19146 

Co-authored-by: Flole998 <[email protected]>