FS-10939 mod_cdr_mongodb: fix format truncation warnings with gcc 7

gcc 7 complains about possible format truncation:

mod_cdr_mongodb.c: In function 'my_on_reporting':
mod_cdr_mongodb.c:242:45: error: '%d' directive output may be truncated writing between 1 and 10 bytes into a region of size 4 [-Werror=format-truncation=]
   snprintf(idx_buffer, sizeof(idx_buffer), "%d", callflow_idx);

The char * idx_buffer has a size of 4 Bytes, and according to gcc's
calculation it is possible that up to 11 Bytes might be copied into it
via int bson_idx.

This commit adds an extra 8 Bytes to char * idx_buffer, which silences
the warnings.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

src/mod/event_handlers/mod_cdr_mongodb/mod_cdr_mongodb.c

@@ -141,7 +141,7 @@ static switch_status_t my_on_reporting(switch_core_session_t *session)
 	bson cdr;
 	int is_b;
 	int bson_idx, callflow_idx;
-	char idx_buffer[4];
+	char idx_buffer[12];
 	char *tmp;
 
 	if (globals.shutdown) {