Fix GH-20674 mb_decode_mimeheader does not handle separator

`?=  =?` is skipped if long term, so skip space character.

Author: youkidearitai

ext/mbstring/mbstring.c

@@ -6395,7 +6395,7 @@ static zend_string* mb_mime_header_decode(zend_string *input, const mbfl_encodin
 					p = temp;
 					/* Decoding of MIME encoded word was successful;
 					 * Try to collapse a run of whitespace */
-					if (p < e && (*p == '\n' || *p == '\r')) {
+					if (p < e && (*p == '\n' || *p == '\r' || *p == ' ')) {
 						do {
 							p++;
 						} while (p < e && (*p == '\n' || *p == '\r' || *p == '\t' || *p == ' '));

ext/mbstring/tests/gh20674.phpt

@@ -0,0 +1,12 @@
+--TEST--
+GH-20674 (mb_decode_mimeheader does not handle separator)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+
+$subject = '=?us-ascii?Q?The_PH?=  =?us-ascii?Q?P_8.5?=';
+var_dump(mb_decode_mimeheader($subject));
+?>
+--EXPECTF--
+string(11) "The PHP 8.5"