（MinGW-GCC8.1）Fix unused variables caused by FOONATHAN_MEMORY_ASSERT_MSG is not d…

[chinanewer]

When I compile the code using mingw64 (GCC 8.1), the following error occurs:

.../src/virtual_memory.cpp
.../src/virtual_memory.cpp: In function 'void foonathan::memory::virtual_memory_release(void*, std::size_t)':
.../src/v0.7-3-8e7bcb82b7.clean/src/virtual_memory.cpp:50:10: error: unused variable 'result' [-Werror=unused-variable]
     auto result = VirtualFree(pages, 0u, MEM_RELEASE);
          ^~~~~~
.../src/v0.7-3-8e7bcb82b7.clean/src/virtual_memory.cpp: In function 'void foonathan::memory::virtual_memory_decommit(void*, std::size_t)':
.../src/virtual_memory.cpp:71:10: error: unused variable 'result' [-Werror=unused-variable]
     auto result = VirtualFree(memory, no_pages * virtual_memory_page_size, MEM_DECOMMIT);
          ^~~~~~
cc1plus.exe: all warnings being treated as errors

like issues-183 and pull-184 as mentioned, however, he has only repaired one problem. There are two of the same problems, and here is another.

[foonathan]

Thank you, I've merged it manually.