ASF CI now lacks "findbugs" tool.
ASF CI upgraded gcc and now there are some errors related to the usage of NULL in calls of fprintf.
- disable findbugs on CI ant-based tasks
- use LOG_DEBUG consistently with the rest of code in C client
Please note that on branch-3.5 we are officially using Maven and Spotbugs, ant stuff is there only for compatibility in build, findbugs is not strictly needed to check the goodness of code in branch-3.5.
branch-3.4 is only with ANT, let's treat this problem as a different task