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.

src/regexp_nfa.c

@@ -5664,7 +5664,8 @@ find_match_text(colnr_T startcol, int regstart, char_u *match_text)
 		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

src/testdir/test_regexp_utf8.vim

@@ -558,4 +558,12 @@ func Test_match_char_class_upper()
   bwipe!
 endfunc
 
+func Test_match_invalid_byte()
+  call writefile(0z630a.765d30aa0a.2e0a.790a.4030, 'Xinvalid')
+  new
+  source Xinvalid
+  bwipe!
+  call delete('Xinvalid')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3409,
 /**/
     3408,
 /**/