patch 8.0.0520: using a function pointer while the function is known

Problem:    Using a function pointer instead of the actual function, which we
            know.
Solution:   Change mb_ functions to utf_ functions when already checked for
            Unicode. (Dominique Pelle, closes #1582)
diff --git a/src/misc2.c b/src/misc2.c
index 2ded997..2f9fed5 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -1874,7 +1874,7 @@
     {
 	while (*p != NUL)
 	{
-	    int l = (*mb_ptr2len)(p);
+	    int l = utfc_ptr2len(p);
 
 	    /* Avoid matching an illegal byte here. */
 	    if (utf_ptr2char(p) == c && l > 1)