patch 9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes #12015)
diff --git a/src/buffer.c b/src/buffer.c
index 8fd81be..cb7bdf4 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1971,7 +1971,7 @@
     // May need to set the spell language.  Can only do this after the buffer
     // has been properly setup.
     if (!curbuf->b_help && curwin->w_p_spell && *curwin->w_s->b_p_spl != NUL)
-	(void)did_set_spelllang(curwin);
+	(void)parse_spelllang(curwin);
 #endif
 #ifdef FEAT_VIMINFO
     curbuf->b_last_used = vim_time();