fix(memory share): handle error before assert

If api_ffa_mem_retrieve_req_version_update returns FFA_ERROR,
handle error gracefully instead of failing assert and panicking.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Change-Id: I642414d85bf6081bd4b524b4f8d844ca153552e7
diff --git a/src/api.c b/src/api.c
index df9bfe1..210c604 100644
--- a/src/api.c
+++ b/src/api.c
@@ -3435,12 +3435,13 @@
 	ret = api_ffa_mem_retrieve_req_version_update(
 		retrieve_msg, message_buffer_size, &retrieve_request, &length,
 		ffa_version);
-	assert(retrieve_request != NULL);
 
 	if (ret.func != FFA_SUCCESS_32) {
 		goto out;
 	}
 
+	assert(retrieve_request != NULL);
+
 	if (plat_ffa_memory_handle_allocated_by_current_world(
 		    retrieve_request->handle)) {
 		ret = ffa_memory_retrieve(to_locked, retrieve_request, length,