lavc: factorize ff_{thread_,re,}get_buffer error messages.

Coccinelle profile used:

  @@
  expression r, ctx, f, loglevel, str, flags;
  @@

  -if ((r = ff_get_buffer(ctx, f, flags)) < 0) {
  -    av_log(ctx, loglevel, str);
  -    return r;
  -}
  +if ((r = ff_get_buffer(ctx, f, flags)) < 0)
  +    return r;

  @@
  expression r, ctx, f, loglevel, str;
  @@

  -if ((r = ff_reget_buffer(ctx, f)) < 0) {
  -    av_log(ctx, loglevel, str);
  -    return r;
  -}
  +if ((r = ff_reget_buffer(ctx, f)) < 0)
  +    return r;

  @@
  expression r, ctx, f, loglevel, str, flags;
  @@

  -if ((r = ff_thread_get_buffer(ctx, f, flags)) < 0) {
  -    av_log(ctx, loglevel, str);
  -    return r;
  -}
  +if ((r = ff_thread_get_buffer(ctx, f, flags)) < 0)
  +    return r;

...along with some manual patches for the remaining ones.
diff --git a/libavcodec/mpc7.c b/libavcodec/mpc7.c
index 26969b1..db37f5f 100644
--- a/libavcodec/mpc7.c
+++ b/libavcodec/mpc7.c
@@ -225,10 +225,8 @@
 
     /* get output buffer */
     frame->nb_samples = MPC_FRAME_SIZE;
-    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0) {
-        av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
+    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
         return ret;
-    }
 
     av_fast_padded_malloc(&c->bits, &c->buf_size, buf_size);
     if (!c->bits)