[BUGFIX] respect rootLevel=-1 in exec_foreign_table_where_query

exec_foreign_table_where_query does not interpret the
rootLevel directive correctly.

Resolves: #45703
Releases: 6.1, 6.0, 4.7, 4.6, 4.5
Change-Id: I2a4dd0335ac5373562f287427d5592cfd406e2f9
Reviewed-on: https://review.typo3.org/18417
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack