src: _snprintf() helper functions nul-terminate buffers, if possible

This patch inconditionally initializes the buffer we get to
nul-terminate it, whenever possible. It's a very simple solution to
catch three overly corner cases:

1) snprintf() returns -1, very much unlikely in practise.

2) msg->len == 0: In such case, I would expect this function is never
   called with an empty message, but let's be safe in this case too.

3) If your buffer is empty, it doesn't nul-terminate the buffer.

Change-Id: I97e517f2d98e83894ea707c63489559302ff6bd2

src/osmux.c

@@ -902,6 +902,9 @@ int osmux_snprintf(char *buf, size_t size, struct msgb *msg)
 	int msg_len = msg->len;
 	int ret;
 
+	if (size)
+		buf[0] = '\0';
+
 	while (msg_len > 0) {
 		if (msg_len < sizeof(struct osmux_hdr)) {
 			LOGP(DLMIB, LOGL_ERROR,

src/rtp.c

@@ -200,6 +200,9 @@ int osmo_rtp_snprintf(char *buf, size_t size, struct msgb *msg)
 	uint8_t *payload;
 	int ret, i;
 
+	if (size)
+		buf[0] = '\0';
+
 	rtph = osmo_rtp_get_hdr(msg);
 	if (rtph == NULL)
 		return -1;