Added backup registers boundary checking

Author: akashihi

src/rtc.rs

@@ -471,16 +471,22 @@ impl Rtc {
     ///
     /// The registers retain their values during wakes from standby mode or system resets. They also
     /// retain their value when Vdd is switched off as long as V_BAT is powered.
-    pub fn read_backup_register(&self, register: usize) -> u32 {
-        self.rtc.bkpr[register].read().bits()
+    pub fn read_backup_register(&self, register: usize) -> Option<u32> {
+        if register < 32 {
+            Some(self.rtc.bkpr[register].read().bits())
+        } else {
+            None
+        }
     }
 
     /// Set content of the backup register.
     ///
     /// The registers retain their values during wakes from standby mode or system resets. They also
     /// retain their value when Vdd is switched off as long as V_BAT is powered.
     pub fn write_backup_register(&self, register: usize, value: u32) {
-        unsafe { self.rtc.bkpr[register].write(|w| w.bits(value)) }
+        if register < 32 {
+            unsafe { self.rtc.bkpr[register].write(|w| w.bits(value)) }
+        }
     }
 }