Fix of by one error on encryption of multiples of 16

Function `lw_encrypt` overruns data sizes which are a multiple of
the key length (and thus encrypts whatever comes next in memory.
This fix takes care of the edge case.

lw/lw.c

@@ -1596,7 +1596,10 @@ int lw_encrypt(uint8_t *out, lw_key_t *key)
 	uint8_t A[LW_KEY_LEN];
 
 	uint16_t const over_hang_bytes = key->len%LW_KEY_LEN;
-    int blocks = key->len/LW_KEY_LEN + 1;
+	int blocks = key->len/LW_KEY_LEN;
+	if (over_hang_bytes) {
+		++blocks;
+	}
 
 	memset(A, 0, LW_KEY_LEN);