tests/fate-run: do not attempt to parse tiny_psnrs output if it failed

tests/fate-run: do not attempt to parse tiny_psnrs output if it failed

This avoids confusing syntax errors with awk later

Likely fixes awk errors at:
http://buildd.debian-ports.org/status/fetch.php?pkg=ffmpeg&arch=sparc64&ver=7%3A2.6.2-1&stamp=1428928967

Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Thanks-to: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com> for the link
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit c0d847e457c1ef72843a63853f1135d52b74131e)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>