patch 8.1.0377: xdiff doesn't use the Vim memory allocation functions

Problem:    Xdiff doesn't use the Vim memory allocation functions.
Solution:   Change the xdl_ defines.  Check for out-of-memory.  Rename
            "ignored" to "vim_ignored".

src/mbyte.c

@@ -756,7 +756,7 @@ codepage_invalid:
 		     * where mblen() returns 0 for invalid character.
 		     * Therefore, following condition includes 0.
 		     */
-		    ignored = mblen(NULL, 0);	/* First reset the state. */
+		    vim_ignored = mblen(NULL, 0);  // First reset the state.
 		    if (mblen(buf, (size_t)1) <= 0)
 			n = 2;
 		    else