19192: cpu/esp32: fixes for boot issues and crashes on ESP32 r=kaspar030 a=Flole998
### 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.
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
Just compiling on the ESP32S2 and running it with WiFi caused it to not start anymore, no output, nothing. When Null is written to that null-pointer it hangs.
The second commit fixed an issue/assertion fail that happens when the WiFi connection drops/disconnects.
### Issues/PRs references
Issue was introduced with PR #19146
Co-authored-by: Flole998 <Flole998@users.noreply.github.com>
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