gst-ffmpeg: lavf: compute probe buffer size more reliably.

The previous code computes the offset by reversing the growth
of the allocated buffer size: it is complex and did lead to
inconsistencies when the size limit is reached.

Fix trac ticket #1991.
(cherry picked from commit 03847eb8259291b4ff1bd840bd779d0699d71f96)

Conflicts:
	libavformat/utils.c

Upstream-Status: Backport

Signed-off-by: Yue Tao <yue.tao@windriver.com>

---
 libavformat/utils.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libavformat/utils.c b/libavformat/utils.c
index 7940037..be73c4a 100644
--- a/gst-libs/ext/libav/libavformat/utils.c
+++ b/gst-libs/ext/libav/libavformat/utils.c
@@ -459,7 +459,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
 {
     AVProbeData pd = { filename ? filename : "", NULL, -offset };
     unsigned char *buf = NULL;
-    int ret = 0, probe_size;
+    int ret = 0, probe_size, buf_offset = 0;
 
     if (!max_probe_size) {
         max_probe_size = PROBE_BUF_MAX;
@@ -499,7 +499,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
             score = 0;
             ret = 0;            /* error was end of file, nothing read */
         }
-        pd.buf_size += ret;
+        pd.buf_size = buf_offset += ret;
         pd.buf = &buf[offset];
 
         memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);
-- 
1.7.5.4