fix: avoid over-allocating memory for 8 byte alignment or less

[romange]

Fixes #509

[danielaparker]

Thanks!

Line 147

if (reinterpret_cast<uintptr_t>(ptr) % align == 0)

will fail compilation if ptr is a "fancy pointer". I don't have a test case for that, but you'll see it if you try to compile boost_examples/interprocess_allocator/shared_memory.cpp. I think you need to write it as

q = extension_traits::to_plain_pointer(ptr);
if (reinterpret_cast<uintptr_t>(q) % align == 0) {

Line 157

align_pad = (align-1);

requires a cast to suppress a warning.

[romange]

@danielaparker PTAL

[danielaparker]

Thanks for contributing