Send one empty static block to give enough lookahead for inflate. This takes 10 bits, of which 7 may remain in the bit buffer. The current inflate code requires 9 bits of lookahead. If the last two codes for the previous block (real code plus EOB) were coded on 5 bits or less, inflate may have only 5+3 bits of lookahead to decode the last real code. In this case we send two empty static blocks instead of one. (There are no problems if the previous block is stored or fixed.) To simplify the code, we assume the worst case of last real code encoded on one bit only.
function _tr_align() {
			send_bits(STATIC_TREES << 1, 3);
			send_code(END_BLOCK, StaticTree.static_ltree);

			bi_flush();

			// Of the 10 bits for the empty block, we have already sent
			// (10 - bi_valid) bits. The lookahead for the last real code (before
			// the EOB of the previous block) was thus at least one plus the length
			// of the EOB plus what we have just sent of the empty static block.
			if (1 + last_eob_len + 10 - bi_valid < 9) {
				send_bits(STATIC_TREES << 1, 3);
				send_code(END_BLOCK, StaticTree.static_ltree);
				bi_flush();
			}
			last_eob_len = 7;
		}