Avoid crash on OOM in virnetmessagetest

The virnetmessagetest code did not check for failure to
allocate the message object. This lead to a crash on OOM
in the test suite.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

tests/virnetmessagetest.c

@@ -327,6 +327,9 @@ static int testMessagePayloadDecode(const void *args ATTRIBUTE_UNUSED)
     };
     int ret = -1;
 
+    if (!msg)
+        return -1;
+
     msg->bufferLength = 4;
     if (VIR_ALLOC_N(msg->buffer, msg->bufferLength) < 0)
         goto cleanup;
@@ -476,6 +479,9 @@ static int testMessagePayloadStreamEncode(const void *args ATTRIBUTE_UNUSED)
     };
     int ret = -1;
 
+    if (!msg)
+        return -1;
+
     msg->header.prog = 0x11223344;
     msg->header.vers = 0x01;
     msg->header.proc = 0x666;