coverity/71508: Fix potential null dereference.

Make sure feature pointer is not null before dereferencing.
vim-patch:7.4.335 #1061

Problem:    No digraph for the new rouble sign.
Solution:   Add the digraphs =R and =P.

https://code.google.com/p/vim/source/detail?r=8ad2ecd116021ad5c945426e8bb80d741392b780
Merge pull request #1064 from mraghavan/coverity-22-07

coverity/71508: Fix potential null dereference.