crypto: fix undefined behavior in shamir unbitslice (#1219)

crypto/shamir.c

@@ -48,7 +48,7 @@ static void bitslice(uint32_t r[8], const uint8_t *x, size_t len) {
   for (arr_idx = 0; arr_idx < len; arr_idx++) {
     cur = (uint32_t)x[arr_idx];
     for (bit_idx = 0; bit_idx < 8; bit_idx++) {
-      r[bit_idx] |= ((cur & (1 << bit_idx)) >> bit_idx) << arr_idx;
+      r[bit_idx] |= ((cur >> bit_idx) & 1) << arr_idx;
     }
   }
 }
@@ -61,7 +61,7 @@ static void unbitslice(uint8_t *r, const uint32_t x[8], size_t len) {
   for (bit_idx = 0; bit_idx < 8; bit_idx++) {
     cur = (uint32_t)x[bit_idx];
     for (arr_idx = 0; arr_idx < len; arr_idx++) {
-      r[arr_idx] |= ((cur & (1 << arr_idx)) >> arr_idx) << bit_idx;
+      r[arr_idx] |= ((cur >> arr_idx) & 1) << bit_idx;
     }
   }
 }