core: Fix segfault when invoking 'data get' CLI command

Invoking 'data get /asterisk/core/channeltypes' caused a crash because
of an assumption of a tech's capabilities to be non-NULL. The
'Surrogate' tech, however, does have a NULL capabilities member,
resulting in a crash.

ASTERISK-27108 #close

Change-Id: I2fbe7715681f43d5565d1e1599269468c26b0e0a

main/data.c

@@ -3145,6 +3145,10 @@ int ast_data_add_codecs(struct ast_data *root, const char *node_name, struct ast
 		return -1;
 	}
 
+	if (!cap) {
+		return 0;
+	}
+
 	count = ast_format_cap_count(cap);
 	for (i = 0; i < count; ++i) {
 		struct ast_format *fmt;