Fixed arithmetic overflow in Rust code on platforms where usize is 16 bits wide.

rust/src/lib.rs

@@ -1141,9 +1141,12 @@ impl QrSegment {
 		let mut result: usize = 0;
 		for seg in segs {
 			let ccbits = seg.mode.num_char_count_bits(version);
-			if seg.numchars >= 1 << ccbits {
+			// ccbits can be as large as 16, but usize can be as small as 16
+			if let Some(limit) = 1usize.checked_shl(u32::from(ccbits)) {
+				if seg.numchars >= limit {
 					return None;  // The segment's length doesn't fit the field's bit width
 				}
+			}
 			result = result.checked_add(4 + usize::from(ccbits) + seg.data.len())?;
 		}
 		Some(result)