patch 9.0.1327: cursor in wrong position below line with virtual text below

Problem:    Cursor in wrong position below line with virtual text below ending
            in multi-byte character.
Solution:   When checking for last character take care of multi-byte
            character.
diff --git a/src/charset.c b/src/charset.c
index a8c20f9..d8feb4e 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -1196,7 +1196,7 @@
 		       || (tp->tp_col == MAXCOL
 			   && ((tp->tp_flags & TP_FLAG_ALIGN_ABOVE)
 				? col == 0
-				: (s[0] == NUL || s[1] == NUL)
+				: (s[0] == NUL || s[charlen] == NUL)
 						  && cts->cts_with_trailing)))
 		    && -tp->tp_id - 1 < gap->ga_len)
 	    {