abis_nm: fix signedness error (uint8_t len cannot be negative)

Detected by Smatch
diff --git a/openbsc/src/libbsc/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index 494d4ca..49e86ba 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -776,7 +776,7 @@
 	char seg_buf[256];
 	char *line_buf = seg_buf+2;
 	unsigned char *tlv;
-	uint8_t len;
+	int len;
 
 	oh = (struct abis_om_hdr *) msgb_put(msg, ABIS_OM_FOM_HDR_SIZE);