gsm_utils: fix use buf_len instead of sizeof in osmo_dump_gsmtime_buf

The function osmo_dump_gsmtime_buf gets a pointer *buf and a parameter
buf_len. The pointer *buf is a string buffer and the function places an
\0 at the end of the buffer before it exists. However it uses
sizeof(buf) as part of the index calculation, which is incorrect. Lets
correct this by using buf_len instead.

Change-Id: Id24263aa7c9a53544f1639b6ceb09ce5615d5114
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index f34d9ea..f2bf57b 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -890,7 +890,7 @@
 {
 	snprintf(buf, buf_len, "%06"PRIu32"/%02"PRIu16"/%02"PRIu8"/%02"PRIu8"/%02"PRIu8,
 		 tm->fn, tm->t1, tm->t2, tm->t3, (uint8_t)tm->fn%52);
-	buf[sizeof(buf)-1] = '\0';
+	buf[buf_len-1] = '\0';
 	return buf;
 }