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

gtk/a11y/gtkcellaccessible.c

@@ -401,7 +401,7 @@ gtk_cell_accessible_get_position (AtkTableCell *table_cell,
   gtk_cell_accessible_parent_get_cell_position (GTK_CELL_ACCESSIBLE_PARENT (parent),
                                                 cell,
                                                 row, column);
-  return ((row && *row > 0) || (column && *column > 0));
+  return ((row && *row >= 0) || (column && *column >= 0));
 }
 
 static int