rpmsg: Handle invalid parameters in public API

There are two cases of possible uninitialized pointer usage in the API,
either the parameters themselves are invalid or we're trying to jump to
functions not required to be implemented by all backends.

Suggested-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>