vsrc_buffer: return EAGAIN if no frame is available.

This is not an erroneous condition, do not print a warning.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vsrc_buffer.c

@@ -175,9 +175,7 @@ static int request_frame(AVFilterLink *link)
     if (!av_fifo_size(c->fifo)) {
         if (c->eof)
             return AVERROR_EOF;
-        av_log(link->src, AV_LOG_ERROR,
-               "request_frame() called with no available frame!\n");
-        return AVERROR(EINVAL);
+        return AVERROR(EAGAIN);
     }
     av_fifo_generic_read(c->fifo, &buf, sizeof(buf), NULL);