Segfault with quick_merge_sorter and MSYS2 MinGW-w64 GCC 9.1

[Morwenn]

I just upgraded my compiler to a 64-bit MSYS2 MinGW-w64 GCC 6.1, and most tests in the "distribution" category featuring quick_merge_sort seem to fail. I'm not sure why, but it's definitely worth investigating.

More info: I logged the size prior to the failure and it always happens during the second iteration when the size is 3336 (out of 10000 elements).

[Morwenn]

Apparently the issue seemed to be a codegen issue. I pushed some dirty workaround which should be enough to circumvent the issue while not pessimizing the code for non-random-access iterators.