net/colo-compare.c: Fix incorrect return when input wrong size

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/colo-compare.c

@@ -807,7 +807,7 @@ static int compare_chr_send(CompareState *s,
     }
 
     if (!size) {
-        return 0;
+        return -1;
     }
 
     entry = g_slice_new(SendEntry);