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.

Originally committed as revision 22571 to svn://svn.ffmpeg.org/ffmpeg/trunk

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];