tests/fate-run.sh: Don't overlook errors from md5 tests

The md5 test up until now ignored errors from ffmpeg (the cli) and just
md5'ed whatever ffmpeg has output; while testing scenarios in which
ffmpeg fails has its merits, errors should not be overlooked by default;
doing so also reduces the effectiveness of sanitizers as errors from
them are ignored. This has happened with a memleak in the AV1 decoder.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

tests/fate-run.sh

@@ -158,7 +158,7 @@ md5pipe(){
 md5(){
     encfile="${outdir}/${test}.out"
     cleanfiles="$cleanfiles $encfile"
-    ffmpeg -y "$@" $(target_path $encfile)
+    ffmpeg -y "$@" $(target_path $encfile) || return
     do_md5sum $encfile | awk '{print $1}'
 }