coverity/90712: Remove unnecessary check for NULL.

** CID 90712:  Dereference after null check  (FORWARD_NULL)
/src/nvim/getchar.c: 3654 in check_abbr()

vim_strsave() never returns NULL, so q is never NULL. Removing the
NULL check for q should fix this warning.
Merge pull request #1741 from oni-link/fix_cid_90712

coverity/90712: Remove unnecessary check for NULL.