Initialized some uninitialized variables in rtc_clk.c and ringbuf.c.

Initialized some uninitialized variables in rtc_clk.c and ringbuf.c.

The following 2 compiler warnings are only reproducible when setting:
   OPTIMIZATION_FLAGS = -Ofast

esp-idf/components/soc/esp32/rtc_clk.c:
In function 'rtc_clk_cpu_freq_get':
esp-idf/components/soc/esp32/rtc_clk.c:506:12:
error: 'freq' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
	return freq;

esp-idf/components/esp_ringbuf/ringbuf.c:
In function 'xRingbufferReceiveSplitFromISR':
esp-idf/components/esp_ringbuf/ringbuf.c:934:26:
error: 'pvTempTailItem' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
	*ppvTailItem = pvTempTailItem;

Closes https://github.com/espressif/esp-idf/pull/2878