Fix integer overflow in BitBufferMut::truncate and append_n

Signed-off-by: Sungjin Kim <carp1230@gmail.com>

Author: ksj1230

vortex-buffer/src/bit/buf_mut.rs

@@ -376,7 +376,8 @@ impl BitBufferMut {
             return;
         }
 
-        let end_bit = self.offset + len;
+        let end_bit = self.offset.checked_add(len)
+    		.expect("BitBufferMut::truncate: offset + len overflow");
         let new_len_bytes = end_bit.div_ceil(8);
         self.buffer.truncate(new_len_bytes);
         self.len = len;
@@ -442,7 +443,9 @@ impl BitBufferMut {
             return;
         }
 
-        let end_bit_pos = self.offset + self.len + n;
+        let end_bit_pos = self.offset.checked_add(self.len)
+			.and_then(|v| v.checked_add(n))
+			.expect("BitBufferMut::append_n: offset + len + n overflow");
         let required_bytes = end_bit_pos.div_ceil(8);
 
         // Ensure buffer has enough bytes