MINOR: checks: Stop xform buffers to null-terminated string for tcp-check rules

MINOR: checks: Stop xform buffers to null-terminated string for tcp-check rules

On the input buffer, it was mainly done to call regex_exec() function. But
regex_exec2() can be used instead. This way, it is no more required to add the
terminating null byte. For the output buffer, it was only done for debugging
purpose.