omxh26xenc: fix coverity with frame test

Coverity was complaining with:
Null pointer dereferences  (REVERSE_INULL) Null-checking "frame"
suggests that it may be null, but it has already been
dereferenced on all paths leading to the check.

The frame == NULL has been removed as 'frame' is actively used
in the code above without any change of dereferencing and setting
its value to NULL before the test.

CID: 1461287
6 jobs for !63 with dab_fix_coverity_subframe in 4 minutes and 26 seconds (queued for 1 second)
merge request