Fix error: unused variable 'result'

[felixf4xu]

Hi,

When I build this project (with gcc 14) for this code:

https://github.com/foonathan/memory/blob/016c9fbd61b57ed2058551dcf225c15a0e716cce/src/virtual_memory.cpp#L48-L52

I had this error:

src/virtual_memory.cpp: In function 'void foonathan::memory::virtual_memory_release(void*, std::size_t)': src/virtual_memory.cpp:50:10: error: unused variable 'result' [-Werror=unused-variable] 50 | auto result = VirtualFree(pages, 0u, MEM_RELEASE); | ^~~~~~

it happens when FOONATHAN_MEMORY_DEBUG_ASSERT is not defined and then FOONATHAN_MEMORY_ASSERT_MSG is an empty macro and then result is not used.

This PR is to fix the compiling error.

Fix #183