ARROW-9047: [Rust] Fix a segfault when setting zero bits in a zero-le…

…ngth bitset.

If the mutable bitset is allocated with zero elements it'll have an
address that, when accessed, segfaults. Even if the number of bits (n)
set is zero, it still reads from this invalid address and things crash.

Closes #7360 from maxburke/rust_empty_bitset_fix

Authored-by: Max Burke <max@urbanlogiq.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>

rust/arrow/src/array/builder.rs

@@ -349,7 +349,7 @@ impl BufferBuilderTrait<BooleanType> for BufferBuilder<BooleanType> {
 
     fn append_n(&mut self, n: usize, v: bool) -> Result<()> {
         self.reserve(n)?;
-        if v {
+        if n != 0 && v {
             unsafe {
                 bit_util::set_bits_raw(self.buffer.raw_data_mut(), self.len, self.len + n)
             }