Fix segfault in do_operation handler (ext-decimal 2.0.x)

Bug was introduced in commit b8b4420 ("Alpha implementation of Number
and Rational", Jan 2019) which changed unsupported-operator handling
from return FAILURE (1.x) to throw ArithmeticError + return SUCCESS
with result zval left uninitialized.

The ZEND_TRY_BINARY_OBJECT_OPERATION macro in Zend engine has called
do_operation for all operator types (including CONCAT, BW_OR) since
PHP 5.6 (commit 089f4967997, Oct 2014). When do_operation throws an
exception but returns SUCCESS without initializing result, the VM's
exception handler crashes in zval_delref_p.

This affects all PHP versions (8.2+), not only PHP 8.5.

Fix by setting result to UNDEF before returning SUCCESS for unsupported
operators, so the exception handler safely skips cleanup.

Co-Authored-By: GLM-5.1 <noreply@z.ai>

Author: andypost

src/decimal.c

@@ -397,6 +397,7 @@ static php_decimal_success_t php_decimal_do_operation_handler(zend_uchar opcode,
     /* Unsupported op type - return success to avoid casting. */
     if (UNEXPECTED(op == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }
 

src/number.c

@@ -105,6 +105,7 @@ static php_decimal_success_t php_decimal_number_do_operation(zend_uchar opcode,
     /* Unsupported operator - return success to avoid casting. */
     if (UNEXPECTED(func == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }
 

src/rational.c

@@ -316,6 +316,7 @@ static php_decimal_success_t php_decimal_rational_do_operation(zend_uchar opcode
     /* Unsupported operator - return success to avoid casting. */
     if (UNEXPECTED(op == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }