libs: encoder: continue if roi meta is NULL

Coverity scan bug:

If the function actually returns a null value, a null pointer
dereference will occur.

In gst_vaapi_encoder_ensure_param_roi_regions(): Return value of
function which returns null is dereferenced without checking
10 jobs for continue-if-roi-null in 35 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #279328
gst indent

00:00:31

passed #279327
manifest

00:00:34

 
  Build
passed #279329
build fedora x86_64

00:10:10

passed #279330
build nodebug fedora x86_64

00:04:56

 
  Test
passed #279331
check fedora

00:03:47

 
  Full Builds
passed #279333
build cerbero android universal

00:12:01

passed #279334
build cerbero cross win32

00:08:05

passed #279335
build cerbero cross win64

00:07:46

passed #279332
build cerbero fedora x86_64

00:05:30

 
  Apps
passed #279336
android universal examples

00:08:38