Fix not handling invalid base64
Fix not handling invalid base64

This patch fixes the following error when a string is not base64:
==00:00:01:37.226 5089== Conditional jump or move depends on uninitialised value(s)
==00:00:01:37.226 5089==    at 0x111E7B: parse_json2data (parser.c:97)
==00:00:01:37.226 5089==    by 0x112A5C: parser_jso_setdata_to_msg (parser.c:567)
==00:00:01:37.226 5089==    by 0x10EB27: downstream_callback (msg.c:315)

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source