msgb: Check the return value of msgb_alloc (Coverity)

In some places, the return value of msgb_alloc/msgb_alloc_headroom
is not checked before it is dereferenced.

This commit adds NULL checks to return with -ENOMEM from the calling
functions if the alloc function has failed.

Fixes: Coverity CID 1249692, 1293376
Sponsored-by: On-Waves ehf
2 files changed