a11y atspi: Fix reporting table cell pos at index (0,0)

Table (cell) row and column indices both start at 0, so an index of 0 is valid.

Adapt the check accordingly and check for non-negative indices instead of positive ones.

(gtk_cell_accessible_parent_get_cell_position sets -1 in the fallback case, so that's still handled as it used to be.)

This fixes reporting the position of the table cell at index (0,0) via AT-SPI.

Fixes: #5161 (closed)