Fix GH-10648: add check function pointer into mbfl_encoding

Previously, mbstring used the same logic for encoding validation as for
encoding conversion.

However, there are cases where we want to use different logic for validation
and conversion. For example, if a string ends up with missing input
required by the encoding, or if a character is input that is invalid
as an encoding but can be converted, the conversion should succeed and
the validation should fail.

To achieve this, a function pointer mb_check_fn has been added to
struct mbfl_encoding to implement the logic used for validation.
Also, added implementation of validation logic for UTF-7, UTF7-IMAP,
ISO-2022-JP and JIS.

Author: pakutoma

UPGRADING

@@ -218,6 +218,16 @@ PHP 8.2 UPGRADE NOTES
     dba_fetch(string|array $key, $skip, $dba): string|false
     is still accepted, but it is recommended to use the new standard variant.
 
+- MBString
+  . mb_check_encoding() now checks input encoding more strictly.
+  . mb_detect_encoding() now checks input encoding more strictly
+    when strict detection is enabled.
+  . mb_convert_encoding() checks the input encoding more strictly
+    if multiple encodings are passed to from_encoding
+    and the mbstring.strict_detection INI directive is set to 1.
+    This change only affects the encoding selection,
+    not the result of the conversion.
+
 - Random
   . random_bytes() and random_int() now throw \Random\RandomException on CSPRNG failure.
     Previously a plain \Exception was thrown.

ext/mbstring/libmbfl/filters/mbfilter_7bit.c

@@ -64,7 +64,8 @@ const mbfl_encoding mbfl_encoding_7bit = {
 	&vtbl_7bit_wchar,
 	&vtbl_wchar_7bit,
 	mb_7bit_to_wchar,
-	mb_wchar_to_7bit
+	mb_wchar_to_7bit,
+	NULL
 };
 
 #define CK(statement)	do { if ((statement) < 0) return (-1); } while (0)