Remove a panic call on a condition that actually can happen.

This can happen if, for example, a blockfn removes itself (leading to
cblockfns < c), or if a blockfn in slot S1 registers another blockfn
that happens to get slot S2 which was scanned before S1 (and thus
doesn't affect c even though it does affect cblockfns, leading to
cblockfns > c).