Mozilla #520500: Allow NULL-value comments
Mozilla #520500: Allow NULL-value comments

This commit changes the policy of fs_comment_validate_byname() to
allow NULL-value comments, and changes fs_comment_new() to handle
the case where value=NULL.

This commit should pass "make check", including the policy change
introduced in the previous commit.

kfish/libfishsound,kfish/libfishsound