oggmux: Fix a maybe-uninitialized warning

Arch Linux, GCC 8.2.1+20181127, CFLAGS='-Og -g3'
4 jobs for Og-uninitialized in 10 minutes and 36 seconds (queued for 19 seconds)
Status Job ID Name Coverage
  Preparation
passed #57191
gst indent

00:00:36

passed #57190
manifest

00:00:24

 
  Build
passed #57192
build fedora x86_64

00:04:43

 
  Full Builds
passed #57193
build android arm64 api28

00:04:51