CLEANUP: log: deinitialization of the log buffer in one function

CLEANUP: log: deinitialization of the log buffer in one function

In several places in the source, there was the same block of code that was
used to deinitialize the log buffer.  There were even two functions that
did this, but they were called only from the code that is in the same
source file (free_tcpcheck_fmt() in src/tcpcheck.c and free_logformat_list()
in src/proxy.c - they were both static functions).

The function free_logformat_list() was moved from the file src/proxy.c to
src/log.c, and a check of the list before freeing the memory was added to
that function.