Initialize m_istamp_id in lookahead::init

[jamesjer]

A build with -fsanitize=address complains that the branch in lookahead::inc_istamp on line 128 of src/sat/sat_lookahead.cpp depends on an uninitialized value. Valgrind makes the same complaint. Code inspection shows that m_istamp_id is read and incremented, but never initialized. Initialize it in lookahead::init at the same time that m_bstamp_id is initialized.