fate-h264-conformance-frext-hpcafl_bcrm_c failes with THREADS=3

[Carl Eugen Hoyos]

Originally reported by Lauri Kasanen:
​http://ffmpeg.org/pipermail/ffmpeg-devel/2018-November/236620.html
fate with THREADS=3 (and higher) fails (on Linux Power8) since bdbbb8f11edbf10add874508c5125c174d8939be, output looks different every time.

$ ./ffmpeg
ffmpeg version N-92534-g5a76171 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 8.2.1 (GCC) 20180813 (Advance-Toolchain-at12.0) [revision 263510]
  configuration: --enable-gpl --cc=/opt/at12.0/bin/gcc
  libavutil      56. 24.101 / 56. 24.101
  libavcodec     58. 40.100 / 58. 40.100
  libavformat    58. 23.100 / 58. 23.100
  libavdevice    58.  6.100 / 58.  6.100
  libavfilter     7. 46.100 /  7. 46.100
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...

Use -h to get full help or, even better, run 'man ffmpeg'
$ make THREADS=3 SAMPLES=../fate-suite fate-h264-conformance-frext-hpcafl_bcrm_c
TEST    h264-conformance-frext-hpcafl_bcrm_c
--- /home/cehoyos/Projects/FFmpeg/tests/ref/fate/h264-conformance-frext-hpcafl_bcrm_c   2018-11-27 01:07:22.736918154 +0000
+++ tests/data/fate/h264-conformance-frext-hpcafl_bcrm_c        2018-11-27 01:16:26.170503653 +0000
@@ -178,7 +178,7 @@
 0,        172,        172,        1,   152064, 0x1c641176
 0,        173,        173,        1,   152064, 0xf71489a4
 0,        174,        174,        1,   152064, 0xd7ac5555
-0,        175,        175,        1,   152064, 0xb4609c6d
+0,        175,        175,        1,   152064, 0x99899c70
 0,        176,        176,        1,   152064, 0xf5b2bd5e
 0,        177,        177,        1,   152064, 0x9f43ce57
 0,        178,        178,        1,   152064, 0x77642dd3
@@ -280,7 +280,7 @@
 0,        274,        274,        1,   152064, 0xa5496a0c
 0,        275,        275,        1,   152064, 0x84c25549
 0,        276,        276,        1,   152064, 0x96691600
-0,        277,        277,        1,   152064, 0x423135db
+0,        277,        277,        1,   152064, 0x3f6e35db
 0,        278,        278,        1,   152064, 0x8d2e08b6
 0,        279,        279,        1,   152064, 0xaeb4c840
 0,        280,        280,        1,   152064, 0xf3e71780
Test h264-conformance-frext-hpcafl_bcrm_c failed. Look at tests/data/fate/h264-conformance-frext-hpcafl_bcrm_c.err for details.
make: *** [fate-h264-conformance-frext-hpcafl_bcrm_c] Error 1

The commit that introduced the regression fixed ticket #4893, the sample in ​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4893/ is not supposed to stop playback / transcoding after 15 (of 30) seconds.