Bug #30669493: ASSERTION `LEN > 0' FAILED IN MY_MEM_COMPARE CALLED IN SORT FROM FILESORT_BUFFER::SORT_BUFFER

Make my_mem_compare() robust against zero-length inputs. Port the
filesort compare benchmarks to the microbenchmark framework and add
a new benchmark to show that the new version is no slower than the
old one (GCC 9.2.1, Coffee Lake 3.2 GHz, optimized mode):

  BM_StdSortCompare0                             1359 iterations     736505 ns/iter
  BM_StdStableSortCompare0                       3039 iterations     300930 ns/iter
  BM_StdSortCompare0ZeroSafe                     1426 iterations     696130 ns/iter
  BM_StdStableSortCompare0ZeroSafe               3780 iterations     263332 ns/iter

Change-Id: Ib600ea6c892962282317c5053a18ec78333463bd