Fix for signedness compiler warning

[shikokuchuo]

Eliminates the following compiler warning:

whisper.cpp:3868:55: warning: comparison of integer expressions of different signedness: ‘std::vector<whisper_full(whisper_context*, whisper_full_params, const float*, int)::beam_candidate>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
 3868 |                         while (beam_candidates.size() > cur_c && beam_candidates[cur_c].sequence.sum_logprobs_all == cur.sequence.sum_logprobs_all && i > 0) {
      |                                ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~

gcc 11.3.0 on Ubuntu 22.04 using cmake.