ext/readline: Fix non-ASCII completion append characters in readline_info() (#24263)

The first byte of completion_append_character could become negative when
plain char is signed. Subsequent readline_info() calls would then use an
invalid index into the one-character string table and could crash.

Read the byte as unsigned char to keep the value within the valid range.
Add a regression test covering ASCII, non-ASCII, and empty strings.

Closes #24263

NEWS

@@ -49,6 +49,10 @@ PHP                                                                        NEWS
 - Phpdbg:
   . Fix wrong command names. (ndossche, Weilin Du)
 
+- Readline:
+  . Fixed a crash in readline_info() when completion_append_character
+    contains a non-ASCII byte. (Weilin Du)
+
 - SOAP:
   . Fixed use of uninitialized func in do_request() on OOM bailout.
     (David Carlier)

ext/readline/readline.c

@@ -248,7 +248,7 @@ PHP_FUNCTION(readline_info)
 				if (!try_convert_to_string(value)) {
 					RETURN_THROWS();
 				}
-				rl_completion_append_character = (int)Z_STRVAL_P(value)[0];
+				rl_completion_append_character = (unsigned char)Z_STRVAL_P(value)[0];
 			}
 			RETVAL_INTERNED_STR(
 				oldval == 0 ? ZSTR_EMPTY_ALLOC() : ZSTR_CHAR(oldval));

ext/readline/tests/readline_info_completion_append_character.phpt

@@ -0,0 +1,27 @@
+--TEST--
+readline_info(): completion_append_character supports non-ASCII bytes
+--EXTENSIONS--
+readline
+--SKIPIF--
+<?php
+if (READLINE_LIB !== 'readline') die('skip readline only');
+?>
+--FILE--
+<?php
+readline_info('completion_append_character', '');
+foreach (["\x7f", "\x80", "\xfftail", ''] as $value) {
+    $old = readline_info('completion_append_character', $value);
+    $current = readline_info('completion_append_character');
+    $info = readline_info();
+    printf("old=%s current=%s info=%s\n",
+        bin2hex($old),
+        bin2hex($current),
+        bin2hex($info['completion_append_character'])
+    );
+}
+?>
+--EXPECT--
+old= current=7f info=7f
+old=7f current=80 info=80
+old=80 current=ff info=ff
+old=ff current= info=