Avoid MSVC unused variable warning in FastZPP

MSVC doesn't support __attribute__((unused)), so this can cause
a lot of warnings for extensions. Use the (void) trick instead.
However, this requires us to initialize the variable as well,
to ensure that ubsan does not read a trap representation.

Author: nikic

Zend/zend_API.h

@@ -1282,7 +1282,7 @@ ZEND_API ZEND_COLD void zend_argument_value_error(uint32_t arg_num, const char *
 		zval *_real_arg, *_arg = NULL; \
 		zend_expected_type _expected_type = Z_EXPECTED_LONG; \
 		char *_error = NULL; \
-		ZEND_ATTRIBUTE_UNUSED zend_bool _dummy; \
+		zend_bool _dummy = 0; \
 		zend_bool _optional = 0; \
 		int _error_code = ZPP_ERROR_OK; \
 		((void)_i); \
@@ -1291,6 +1291,7 @@ ZEND_API ZEND_COLD void zend_argument_value_error(uint32_t arg_num, const char *
 		((void)_expected_type); \
 		((void)_error); \
 		((void)_optional); \
+		((void)_dummy); \
 		\
 		do { \
 			if (UNEXPECTED(_num_args < _min_num_args) || \