common BUGFIX avoid overflow

... even though it's harmless.
diff --git a/src/ly_common.c b/src/ly_common.c
index e2293bc..766faf8 100644
--- a/src/ly_common.c
+++ b/src/ly_common.c
@@ -74,7 +74,8 @@
 LY_ERR
 ly_strntou8(const char *nptr, size_t len, uint8_t *ret)
 {
-    uint8_t num = 0, dig, dec_pow;
+    uint8_t num = 0, dig;
+    uint16_t dec_pow;
 
     if (len > 3) {
         /* overflow for sure */