resulttool/resultutils: allow index generation despite corrupt json

non-release indexes will continue to generate when test output is
corrupted.

Signed-off-by: Michael Halstead <mhalstead@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>