stun: Explicitly avoid a memcpy() from NULL

stun: Explicitly avoid a memcpy() from NULL

If stun_message_append_bytes() is called through
stun_message_append_flag(), data will be NULL and len will be 0. This
will result in a memcpy(ptr, NULL, 0) call. This probably won’t do any
harm (since any reasonable memcpy() implementation will immediately
return if (len == 0)), but the standard allows for memcpy() to explode
if (data == NULL), regardless of the value of len.

In order to be conformant, and to shut up the scan-build static analysis
warning about it, only do the memcpy() if (len > 0).