msgb: Fix -Wsign-compare warnings

Change-Id: I4bd14bb712fb3868b55d7e79d13276b707a6cdcb
diff --git a/src/msgb.c b/src/msgb.c
index ccb55aa..0881a55 100644
--- a/src/msgb.c
+++ b/src/msgb.c
@@ -426,11 +426,11 @@
  */
 char *msgb_hexdump_buf(char *buf, size_t buf_len, const struct msgb *msg)
 {
-	int buf_offs = 0;
+	unsigned int buf_offs = 0;
 	int nchars;
 	const unsigned char *start = msg->data;
 	const unsigned char *lxhs[4];
-	int i;
+	unsigned int i;
 
 	lxhs[0] = msg->l1h;
 	lxhs[1] = msg->l2h;