Placing block in offhand doesn't work if block in mainhand can't be placed.

To reproduce: Put down two regular blocks next to each other, put a slab on one of them, stand on the slab and partially on the air above the other block, having a regular block in your mainhand and a slab in your offhand. Try to place the slab under you.

Expected result: Regular block placement from mainhand fails, slab gets placed from offhand.

Actual result: Nothing.