consistently check the result of osmo_shift_v_fixed()

Coverity points out we forgot to check the return value of
osmo_shift_v_fixed() in some places. Add checks which verify
the expected length of data which is skipped by the parser.

Change-Id: I20406f411810e966443d6fd5a4620b9a66cd9809
Related: CID#135160
1 file changed