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

[andypost]

Summary

• The do_operation handler segfaults when encountering unsupported opcodes (ZEND_CONCAT, ZEND_BW_OR, etc.)
• The handler throws ArithmeticError and returns SUCCESS without initializing the result zval, causing the VM's exception handler to crash in zval_delref_p
• Fix by adding ZVAL_UNDEF(result) before returning SUCCESS for unsupported operators

Root Cause

This bug was introduced in commit b8b4420 (Jan 2019, "Alpha implementation of Number and Rational") when the unsupported-operator handling was changed from the 1.x behavior:

• 1.x: return FAILURE; — engine falls through to type juggling (casting object to string/int)
• 2.0.x: throws ArithmeticError + return SUCCESS + result left uninitialized — engine skips type juggling but crashes cleaning up the garbage result zval

This affects all PHP versions (8.2+), not only PHP 8.5. The concat_function → do_operation(ZEND_CONCAT) call path has existed since PHP 5.6 (commit 089f4967997, Oct 2014). The ZEND_TRY_BINARY_OBJECT_OPERATION macro is identical between PHP 8.4 and 8.5.

Test Plan

• All 131 tests pass on PHP 8.2, 8.3, 8.4, 8.5 (previously 3 tests failed with SIGSEGV)
• Decimal/operators.phpt, Number/operators.phpt, Rational/operators.phpt now pass
• Verified no regression on supported operators (+, -, *, /, %, **, <<, >>)

[andypost]

Using it to make pass https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/100779

[andypost]

This is not a PHP 8.5-only issue

After further investigation, the concat_function → do_operation(ZEND_CONCAT) call path has existed since PHP 5.6 (commit 089f4967997, Oct 2014 — "Moved proxy object support in ASSIGN_ADD (and family) from VM to slow paths of corresponding operators"). The ZEND_TRY_BINARY_OBJECT_OPERATION macro and how concat_function uses it are identical between PHP 8.4 and 8.5.

The bug is in ext-decimal 2.0.x, introduced in commit b8b4420 (Jan 2019, "Alpha implementation of Number and Rational") when the unsupported-operator handling was changed from the 1.x behavior:

	1.x (correct)	2.0.x (buggy)
Unsupported opcode	return FAILURE;	throws ArithmeticError + return SUCCESS + result left uninitialized
Engine behavior	Falls through to type juggling (cast object to string/int)	Skips type juggling, but crashes in zval_delref_p cleaning up garbage result zval

The ZVAL_UNDEF(result) fix is correct and needed on all PHP versions (5.6+) when using ext-decimal 2.0.x.

[andypost]

Tested other branches and since PHP 8.2 all extension needs this patch

[rtheunissen]

Thanks @andypost!