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/message.c b/src/message.c
index a4dd746..a9a6669 100644
--- a/src/message.c
+++ b/src/message.c
@@ -315,7 +315,7 @@
 	for (;;)
 	{
 	    do
-		half = half - (*mb_head_off)(s, s + half - 1) - 1;
+		half = half - utf_head_off(s, s + half - 1) - 1;
 	    while (half > 0 && utf_iscomposing(utf_ptr2char(s + half)));
 	    n = ptr2cells(s + half);
 	    if (len + n > room || half == 0)