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

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.

Bug: https://gitlab.gnome.org/GNOME/gtk/-/issues/5161
Forwarded: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6173
Origin: 3.24.39, commit:89517775183fa17f77c2cfc413dec86e751c3496

Gbp-Pq: Name a11y-atspi-Fix-reporting-table-cell-pos-at-index-0-0.patch