handle empty ranges in BitSlice::copy_within

resolves ferrilab#236

src/slice/api.rs

@@ -2229,6 +2229,12 @@ where
 	where R: RangeExt<usize> {
 		let len = self.len();
 		let src = src.normalize(0, len);
+        // prevent the asserts below from failing for empty ranges
+        if src.start == src.end {
+            assert!(src.start <= len, "range start index {} is out of bounds for bit-slice of length {}", src.start, len);
+            assert!(dest <= len, "index {} is out of bounds for bit-slice of length {}", dest, len);
+            return;
+        }
 		self.assert_in_bounds(src.start, 0 .. len);
 		self.assert_in_bounds(src.end, 0 ..= len);
 		self.assert_in_bounds(dest, 0 .. len);