ffmpeg: fix deadlock regression in threading error handing

Commit fc9c857c introduced deadlock regression when processing too many inputs:
  ffmpeg $(seq -f " -f lavfi -i aevalsrc=0:d=%.0f" 70) -vf concat=n=70:v=0:a=1 -f null -
Happens for different number of inputs, depending on available memory size,
overcommit settings, ulimits, etc. Easily noticeable for 32-bit builds,
that exhaust address space allocating 8-10 MB stack for each thread.
Earlier ffmpeg versions exited with unhelpful "Conversion failed!" message.

This patch fixes both problems: it frees the queue to prevent deadlock
and adds a meaningful error message if pthread_create() fails.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -3237,8 +3237,11 @@ static int init_input_threads(void)
         if (ret < 0)
             return ret;
 
-        if ((ret = pthread_create(&f->thread, NULL, input_thread, f)))
+        if ((ret = pthread_create(&f->thread, NULL, input_thread, f))) {
+            av_log(NULL, AV_LOG_ERROR, "pthread_create failed: %s. Try to increase `ulimit -v` or decrease `ulimit -s`.\n", strerror(ret));
+            av_thread_message_queue_free(&f->in_thread_queue);
             return AVERROR(ret);
+        }
     }
     return 0;
 }