The test currently checks that 1-byte is allocated when malloc(0) is
called, by dereferencing the pointer.
https://github.com/llvm/llvm-project/pull/155943 changed ASan to
consider the dereference to be a heap buffer overflow. This patch
changes the test to check the allocated size is still 1-byte, but not
dereference the pointer.
This aims to fix the breakage reported in
https://github.com/llvm/llvm-project/pull/155943#issuecomment-3239543505
It also enables the test for 64-bit Windows.
6dfd8d0ab4