rmt: Fix LoadProhibited on RX END

rmt: Fix LoadProhibited on RX END

Fixes exceptions LoadProhibited, when rmt tries read variable p_rmt on RX END which can be 0.
It happens after esp_reset()  (OTA) and the rmt is not probably stopped by it.
In other words the routine rmt_driver_isr_default was called before second rx channel was registered.

scenario:
register tx channel
register rx channel
reboot
register tx channel
-> exception (because rx channel is accessed)

Merges https://github.com/espressif/esp-idf/pull/1671