Don't crash when passing a NULL message to __astman_get_header.

Before this commit, __astman_get_header would blindly dereference the passed in
'struct message *' to traverse the header list.  There are cases, however, such
as '*CLI> sip qualify peer foo' where the message pointer is NULL, so we need
to check for that.
........

Merged revisions 373131 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@373132 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/manager.c

@@ -1831,6 +1831,10 @@ static const char *__astman_get_header(const struct message *m, char *var, int m
 	int x, l = strlen(var);
 	const char *result = "";
 
+	if (!m) {
+		return result;
+	}
+
 	for (x = 0; x < m->hdrcount; x++) {
 		const char *h = m->headers[x];
 		if (!strncasecmp(var, h, l) && h[l] == ':') {