misc: Fix warnings on Cerbero's ancient MinGW

gesdemux.c:297:3: error: value computed is not used [-Werror=unused-value]
8 jobs for master in 15 minutes and 20 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Preparation
passed #99866
gst indent

00:00:26

passed #99865
manifest

00:00:17

 
  Build
passed #99867
build fedora x86_64

00:02:38

 
  Test
passed #99868
check fedora

00:03:30

 
  Full Builds
passed #99869
build android arm64 api28

00:02:50

passed #99871
build cerbero cross win32

00:05:26

passed #99872
build cerbero cross win64

00:05:44

passed #99870
build cerbero fedora x86_64

00:06:11