UnlockedFlash::program can write out of bounds

[sjorsdewit]

It seems the function UnlockedFlash::program() can write outside the bounds of the flash memory if given an invalid offset:

stm32f4xx-hal/src/flash.rs

Lines 239 to 241 in 6d0c292

	unsafe {
	ptr::write_volatile(ptr.add(offset), *byte);
	}

One way to improve this might be to check offset against self.len() each iteration, or maybe there is a way to pre-compute if the offset + length of the iterator exceeds the flash length?