master: part of the mbstring bailout fix got lost in a merge #24489

The PHP-8.6 into master merge 9705fda dropped this hunk from mb_convert_buf_ensure() in ext/mbstring/libmbfl/mbfl/mbfl_encoding.h. It also dropped the test ext/mbstring/tests/oom_buf_growth.phpt:

 		if (!newstr) {
+			/* An earlier growth may have moved the buffer. */
+			buf->out = *out;
+			buf->limit = *limit;
 			return FAILURE;
 		}

Both are on PHP-8.6 but missing on master. git diff origin/master origin/PHP-8.6 -- ext/mbstring/libmbfl/mbfl/mbfl_encoding.h shows only this hunk, and git show --remerge-diff 9705fda1bb -- ext/mbstring shows the merge removing it. I'm guessing it was an accident during conflict resolution. Re-applying the three lines and restoring the test from PHP-8.6 should be all that's needed.