unsafe improvements (#6551)

* Remove unnecessary use of `unsafe` by reusing existing code

* Remove unnecessary use of MaybeUninit

arrow-buffer/src/builder/offset.rs

@@ -70,8 +70,11 @@ impl<O: ArrowNativeType> OffsetBufferBuilder<O> {
     ///
     /// Panics if offsets overflow `O`
     pub fn finish_cloned(&self) -> OffsetBuffer<O> {
-        O::from_usize(self.last_offset).expect("overflow");
-        unsafe { OffsetBuffer::new_unchecked(self.offsets.clone().into()) }
+        let cloned = Self {
+            offsets: self.offsets.clone(),
+            last_offset: self.last_offset,
+        };
+        cloned.finish()
     }
 }
 

arrow-buffer/src/util/bit_mask.rs

@@ -127,16 +127,16 @@ unsafe fn set_upto_64bits(
 }
 
 /// # Safety
-/// The caller must ensure all arguments are within the valid range.
+/// The caller must ensure `data` has `offset..(offset + 8)` range, and `count <= 8`.
 #[inline]
 unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
     debug_assert!(count <= 8);
-    let mut tmp = std::mem::MaybeUninit::<u64>::new(0);
+    let mut tmp: u64 = 0;
     let src = data.as_ptr().add(offset);
     unsafe {
-        std::ptr::copy_nonoverlapping(src, tmp.as_mut_ptr() as *mut u8, count);
-        tmp.assume_init()
+        std::ptr::copy_nonoverlapping(src, &mut tmp as *mut _ as *mut u8, count);
     }
+    tmp
 }
 
 /// # Safety