Fix #81693: mb_check_encoding(7bit) segfaults

`php_mb_check_encoding()` now uses conversion to `mbfl_encoding_wchar`.
Since `mbfl_encoding_7bit` has no `input_filter`, no filter can be
found.  Since we don't actually need to convert to wchar, we encode to
8bit.

Closes GH-7712.

NEWS

@@ -2,6 +2,9 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? ????, PHP 8.1.2
 
+- MBString:
+  . Fixed bug #81693 (mb_check_encoding(7bit) segfaults). (cmb)
+
 - Reflection:
   . Fixed bug #81681 (ReflectionEnum throwing exceptions). (cmb)
 

ext/mbstring/libmbfl/mbfl/mbfl_convert.c

@@ -307,7 +307,8 @@ const struct mbfl_convert_vtbl* mbfl_convert_filter_get_vtbl(const mbfl_encoding
 		from = &mbfl_encoding_8bit;
 	} else if (from->no_encoding == mbfl_no_encoding_base64 ||
 			   from->no_encoding == mbfl_no_encoding_qprint ||
-			   from->no_encoding == mbfl_no_encoding_uuencode) {
+			   from->no_encoding == mbfl_no_encoding_uuencode ||
+			   from->no_encoding == mbfl_no_encoding_7bit) {
 		to = &mbfl_encoding_8bit;
 	}
 

ext/mbstring/tests/bug81693.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Bug #81693 (mb_check_encoding(7bit) segfaults)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+var_dump(mb_check_encoding('Hello world', '7bit'));
+?>
+--EXPECT--
+bool(true)