vty: Fix misusage of snprintf in vty/utils.c

Compiled with ubuntu 1204 (precise), where -Wformat-security is enabled by
-Wall.

Test yields ok, but the current implementation doesn't properly support
multi-character separators and end strings. So the test output is truncated.

Addresses:
utils.c: In function 'vty_cmd_string_from_valstr':
utils.c:84:2: warning: format not a string literal and no format arguments [-Wformat-security]
utils.c:84:2: warning: format not a string literal and no format arguments [-Wformat-security]
utils.c:108:2: warning: format not a string literal and no format arguments [-Wformat-security]
utils.c:108:2: warning: format not a string literal and no format arguments [-Wformat-security]
diff --git a/.gitignore b/.gitignore
index 2ed0144..da36165 100644
--- a/.gitignore
+++ b/.gitignore
@@ -72,6 +72,7 @@
 tests/loggingrb/loggingrb_test
 tests/ringbuf/ringbuf_test
 tests/strrb/strrb_test
+tests/vty/vty_test
 
 utils/osmo-arfcn
 utils/osmo-auc-gen