patch 8.2.3409: reading beyond end of line with invalid utf-8 character

Problem:    Reading beyond end of line with invalid utf-8 character.
Solution:   Check for NUL when advancing.
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index 9757d7c..c7db981 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -5664,7 +5664,8 @@
 		match = FALSE;
 		break;
 	    }
-	    len2 += MB_CHAR2LEN(c2);
+	    len2 += enc_utf8 ? utf_ptr2len(rex.line + col + len2)
+							     : MB_CHAR2LEN(c2);
 	}
 	if (match
 		// check that no composing char follows