tests/unit/test_xasprintf.c: Fix use of volatile pointer

tests/unit/test_xasprintf.c: Fix use of volatile pointer

volatile needs to be casted away behind a [[gnu::noipa]] function, to
make that invisible to the compiler.  Otherwise, the compiler can see
that it is being discarded, and is free to abuse Undefined Behavior.

Closes: <https://github.com/shadow-maint/shadow/issues/1028>
Reported-by: Chris Hofstaedtler <zeha@debian.org>
Tested-by: Chris Hofstaedtler <zeha@debian.org>
Reviewed-by: Chris Hofstaedtler <zeha@debian.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>