selftests/ftrace: Handle the absence of tput

In environments where tput is not available, we get the following
error
$ ./ftracetest: 163: [: Illegal number:
because ncolors is an empty string. Fix that by setting it to 0 if the
tput command fails.

Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>