utils: use temp var in fallback GST_WRITE_*() macros
utils: use temp var in fallback GST_WRITE_*() macros

To make sure the value is only expanded/used once, in case
there are side effects to it, and to avoid calculating it
or looking it up multiple times if there is a calculation
or lookup involved.

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer