Fix erroneous behaviour when format probe hits end of file

If the format probe hits end of file, do not add the error code
to the buffer position.  This is obviously wrong, and with a
small input file would cause a negative buffer overflow.

Fixes issue 1818.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22571 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/utils.c

@@ -495,6 +495,7 @@ int ff_probe_input_buffer(ByteIOContext **pb, AVInputFormat **fmt,
                 return ret;
             }
             score = 0;
+            ret = 0;            /* error was end of file, nothing read */
         }
         pd.buf_size += ret;
         pd.buf = &buf[offset];