basesrc: ensure submitted buffer list is writable

Fixes flaky appsrc unit test where depending on scheduling
the submitted list might not be writable if submitted via
an action signal from the application thread.

Fixes gst-plugins-base#522
6 jobs for basesrc-fix-flaky-appsrc-bufferslist-unit-test in 30 minutes and 5 seconds
Status Job ID Name Coverage
  Preparation
passed #67319
gst indent

00:00:31

passed #67318
manifest

00:00:36

 
  Build
passed #67320
build fedora x86_64

00:02:57

 
  Test
passed #67321
check fedora

00:19:32

 
  Full Builds
passed #67322
build android arm64 api28

00:04:16

passed #67323
build cerbero fedora x86_64

00:06:58