Turn the non-valgrind code path into inline functions to avoid compiler warnings

Turn the non-valgrind code path into inline functions to avoid compiler warnings

Recent gcc will warn if you have a statement that's just a macro
expanding to (0), but not if you have an inline stub function that
always returns 0, so let's do the latter.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37286
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>