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/regexp.c b/src/regexp.c
index b4fe7d7..e1f6484 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -4732,7 +4732,7 @@
 		    break;
 		}
 		if (enc_utf8)
-		    opndc = mb_ptr2char(opnd);
+		    opndc = utf_ptr2char(opnd);
 		if (enc_utf8 && utf_iscomposing(opndc))
 		{
 		    /* When only a composing char is given match at any
@@ -4741,7 +4741,7 @@
 		    for (i = 0; reginput[i] != NUL;
 						i += utf_ptr2len(reginput + i))
 		    {
-			inpc = mb_ptr2char(reginput + i);
+			inpc = utf_ptr2char(reginput + i);
 			if (!utf_iscomposing(inpc))
 			{
 			    if (i > 0)
@@ -4750,7 +4750,7 @@
 			else if (opndc == inpc)
 			{
 			    /* Include all following composing chars. */
-			    len = i + mb_ptr2len(reginput + i);
+			    len = i + utfc_ptr2len(reginput + i);
 			    status = RA_MATCH;
 			    break;
 			}