check asprintf return value

The current asprintf usage triggers many warnings like:

sslh-main.c: In function 'print_usage':
sslh-main.c:86:17: warning: ignoring return value of 'asprintf',
	declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

probe.c

@@ -63,7 +63,8 @@ int get_num_builtins(void) {
 /* Sets the protocol name to connect to in case of timeout */
 void set_ontimeout(const char* name)
 {
-    asprintf(&on_timeout, "%s", name);
+    int res = asprintf(&on_timeout, "%s", name);
+    CHECK_RES_DIE(res, "asprintf");
 }
 
 /* Returns the protocol to connect to in case of timeout;