If number of DFA states reaches at 1024, all transition tables are
cleared in build_state() in order to avoid out-of-memory.  However, for
initial state that shouldn't be done, because it's always used.

BTW, this patch enables to revert a previous patch "grep: do not count
newline before the start  of buffer", because no longer the code is never
be run through at a first character of a text.