avfilter/vf_libopencv: let the shape file alone define the element size

For CV_SHAPE_CUSTOM the documentation states that cols and rows from the
filter parameters are ignored and the size is taken from the file, but
read_shape_from_file() used the caller supplied values as a baseline:
*rows was incremented and *cols was raised to the file width. Reset both
so that only the file content matters.

This also removes two remaining out-of-bounds accesses in the fill loop,
which are reachable with a custom shape file whose content is empty or
whose last line lacks the terminating newline.

Assisted-by: Fairy

libavfilter/vf_libopencv.c

@@ -136,6 +136,9 @@ static int read_shape_from_file(int *cols, int *rows, int **values, const char *
     if ((ret = av_file_map(filename, &buf, &size, 0, log_ctx)) < 0)
         return ret;
 
+    /* the file alone defines the size of the shape */
+    *cols = *rows = 0;
+
     /* prescan file to get the number of lines and the maximum width */
     w = 0;
     for (i = 0; i < size; i++) {