add null pointer check for msg buffer

Test: Build
Change-Id: I94e08ccc8372449f11f6e673b6449c50d7eef4f9
diff --git a/trusty/keymaster/trusty_keymaster_ipc.cpp b/trusty/keymaster/trusty_keymaster_ipc.cpp
index fbd0eb3..686e7ae 100644
--- a/trusty/keymaster/trusty_keymaster_ipc.cpp
+++ b/trusty/keymaster/trusty_keymaster_ipc.cpp
@@ -55,6 +55,11 @@
 
     size_t msg_size = in_size + sizeof(struct keymaster_message);
     struct keymaster_message* msg = reinterpret_cast<struct keymaster_message*>(malloc(msg_size));
+    if (!msg) {
+        ALOGE("failed to allocate msg buffer\n");
+        return -EINVAL;
+    }
+
     msg->cmd = cmd;
     memcpy(msg->payload, in, in_size);