Fix select_support_mcl::init_fast

[jltsiren]

select_support_mcl::init_fast would sometimes crash because the initialization tried to access the padding after the bitvector (see simongog#402). This PR fixes the issue.

In the example, pos_of_last_arg_in_the_block is 25587424, which is greater than the length of the bitvector (25587416).

The relevant part of the code is here: https://github.com/simongog/sdsl-lite/blob/c32874cb2d8524119f25f3b501526fe692df29f4/include/sdsl/select_support_mcl.hpp#L287-L308

• On line 287, args_in_the_word() may find nonexistent args in the padding, at least for select_support_mcl<0, 0>.
• That could bring the number of args in the last superblock past another multiple of 64, passing the test on line 288.
• In that case, we store the position of a nonexistent arg in array arg_position on line 289.
• If the next multiple of 64 would start another superblock, we have a full superblock and pass the test on line 293.
• On line 295, we take the the nonexistent position we stored in arg_position as a candidate for pos_of_last_arg_in_the_block.
• We don't find further candidates, because the loop on line 297 tests for ii < v->size().
• If the last superblock happens to be long, we pass the test on line 303.
• The loop on line 307 iterates over the superblock, up to and including pos_of_last_arg_in_the_block.
• We access each position in the superblock on line 308, which ultimately causes the assertion failure.

We can fix this by adding cnt_new = std::min(cnt_new, m_arg_cnt); after line 287.

[tsnorri]

If using GPL is not something that is particularly important, would you consider patching the development version of SDSL, too? It uses the revised BSD licence.

[jltsiren]

I got the impression that the development version has also been abandoned.