examples: playback-test: fix crashes when setting buffer-size
playbin's buffer-size property takes a gint, not a gint64, so only pass the bits expected to the vararg function, or the terminator might not be found, leading to crashes, esp. with negative numbers. Spotted by Ravi Kiran K N <ravi.kiran@samsung.com> https://bugzilla.gnome.org/show_bug.cgi?id=729617