trace: fix serialization of const char * tracepoint arguments

The serializer was reading from the destination buffer instead of the
source string. As a result all string arguments appeard as empty
strings in the trace.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>