list: fix an issue with android build using clang

Sorry, I don't understand the android build system enough to say *which*
version of clang this effects, but either "clang-2812033" or
"clang-3016494" (probably the later).

But when 'sample' is undefined (ie. unitialized variable), the result is
not as well defined as it is with gcc.  Instead use a typeof() cast with
a defined value (ie. zero).

This fixes a crash that was reported on android.

Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Rob Clark <robclark@freedesktop.org>