Only erase RTC RAM if sleep mode doesn't look valid (esp8266#619)

Source: http://esp8266.ru/forum/threads/mem-check-fail.168/#post-7354

Author: igrr

cores/esp8266/core_esp8266_phy.c

@@ -261,7 +261,10 @@ void user_rf_pre_init() {
     // *((volatile uint32_t*) 0x60000710) = 0;
 
     volatile uint32_t* rtc_reg = (volatile uint32_t*) 0x60001000;
-    rtc_reg[30] = 0;
+    if((rtc_reg[24] >> 16) > 4) {
+        rtc_reg[24] &= 0xFFFF;
+        rtc_reg[30] = 0;
+    }
 
     system_set_os_print(0);
     __run_user_rf_pre_init();