esp_reset_reason_init would check for reset reason hint, and
unconditionally set RTC_RESET_CAUSE_REG (which is also
RTC_ENTRY_ADDR_REG) to hint value 0, i.e. 0x80000000. However the ROM
code treats this value as valid deep sleep wake stub entry point, and
tries to jump to it.
Clear RTC_RESET_CAUSE_REG only if it contained a valid reset reason
hint, and simply set the register value to 0 when doing so. Also add
a check to esp_get_deep_sleep_wake_stub function that deep sleep wake
stub entry address must be in IRAM.
Reported in https://esp32.com/viewtopic.php?f=13&t=6919.
