Replace removed is_error macro

[jelly]

The json-c library removed the is_error macro in 0.13, replaced the macro calls with a != NULL statement.

[paulrho]

This looks like it is required for the issue of the json build error, however, there may be another change required to bti.c:

-#include <json-c/json.h>
+#include <json/json.h>
...
-PKG_CHECK_MODULES([JSON], [json-c])
+PKG_CHECK_MODULES([JSON], [json])

At least, that's what seems to be altered in the Fedora build...investigating...

So far: seems maybe not - looks like we are ahead of the game (Package json-c changed json.h to json-c.h some time in the past)