patch 8.0.0451: some macros are in lower case

Problem:    Some macros are in lower case.
Solution:   Make a few more macros upper case. Avoid lower case macros use an
            argument twice.
diff --git a/src/popupmnu.c b/src/popupmnu.c
index fc5e1c3..58e3817 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -335,7 +335,7 @@
 		case 3: p = pum_array[idx].pum_extra; break;
 	    }
 	    if (p != NULL)
-		for ( ; ; mb_ptr_adv(p))
+		for ( ; ; MB_PTR_ADV(p))
 		{
 		    if (s == NULL)
 			s = p;
@@ -369,7 +369,7 @@
 					{
 					    size -= has_mbyte
 						    ? (*mb_ptr2cells)(rt) : 1;
-					    mb_ptr_adv(rt);
+					    MB_PTR_ADV(rt);
 					} while (size > pum_width);
 
 					if (size < pum_width)