avfilter/opencl: Fix program_opencl for source code larger than 64kB

The condition (pos < len) is always true and the
rest of the OpenCL program code would not be read, while
the maximum number of "rb" is "len - pos - 1", and then, the
maximum number of the "pos" is "len - 1".

Fixes: trac.ffmpeg.org/ticket/9217

libavfilter/opencl.c

@@ -242,7 +242,7 @@ int ff_opencl_filter_load_program_from_file(AVFilterContext *avctx,
             goto fail;
         }
         pos += rb;
-        if (pos < len)
+        if (pos + 1 < len)
             break;
         len <<= 1;
         err = av_reallocp(&src, len);