Sleep ignores undead creatures as well as creatures immune to being charmed

Quotes from Sleep, PHB p.276:

Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures).

[...]

Undead and creatures immune to being charmed aren't affected by this spell.

Sleep states that creatures within the range are affected, but also states undead aren't affected. Specific beats general so you ignore the undead.

Mike Mearls supports this interpretation:

Radim Havlíček @Tavicz
@mikemearls Spell sleep: If the creature with the lowest current HPs has imunity to sleep, are its HPs subtracted from the total?
_{12:30 AM - 24 Mar 2015}

Mike Mearls @mikemearls
@Tavicz no, just skip it and move to the next
_{5:20 PM - 25 Mar 2015}

The caster can choose any point in range, and try to cast a fireball there.

Whether there is a clear path, or not, is not relevant at this stage : the character is blinded, and can't know if there is one. A blinded character shouldn't be allowed to detect obstacles for free when targeting a point for an area of effect spell. Hence, he can choose any point in the spell's range.

It will most certainly backfire.

V2Blast appropriately reminded of the clear path to the target rule :

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

Obviously, there is an obstruction between you and any point outside the toad : the Giant Toad itself.
The fireball's point of origin will be located on the near side of the toad's belly, an the spell's full effect will remain inside it. Only the toad and the wizard are affected. Probably.

Though very unlikely, the DM might decide there's an occasional clear path somewhere :

• The swallow attack doesn't explicitly grant full cover to people outside. At least some air is passing, as it does not provoke suffocation.
• As Large the creature may be, a fully-geared adventurer isn't a light meal. Parts of it are probably still stuck in the throat of the toad.

He might call for luck here, and allow a (tiny) chance for the spell to resolve as expected by the caster. If not... Not only will the fireball's point of origin be located inside the toad (affecting again the toad and the wizard) - but it will also expand to a 20-foot sphere centered on the toad, as the fireball spreads around corners.

This might be effective - if the caster has some way to mitigate fire damage, and is surrounded by opponents.