gstutils: Fix build with clang -Werror=cast-align
gstutils: Fix build with clang -Werror=cast-align

https://bugzilla.gnome.org/show_bug.cgi?id=755657

centricular/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer