Fix to finalize mac compute when there is not a last message chunk.

	Code implemented in TAs may need to call TEE_MACComputeFinal,
	without last message chunk.

Change-Id: Ida9d6128a2692740d50aae9de6a3a9964a620340
Reviewed-on: https://gerrit.st.com/15246
Tested-by: Thierry BINIGUER <thierry.biniguer@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>
Tested-by: Etienne CARRIERE <etienne.carriere@st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>
This could be written more compact as:
if (data && data_len) ...