[DPLL] Initialize Watch length with # of literals

[hyunsooda]

The DPLL solver crashes due to out-of-bounds access when initialized with the clause length. This occurs when working with a CNF of four variables and three clauses.

c ( ¬A ∨ B ) ∧ ( ¬B ∨ ¬C ) ∧ ( C ∨ ¬D )
p cnf 4 3
-1 2 0
-2 -3 0
3 -4 0

thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 3', src/solver/dpll.rs:30:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR resolves the issue and includes a related test case. Thanks for the good resource. Learned a lot.