Fix segfault when preloading constant AST closure

[iluuu1994]

Fixes GH-21059

Note: This is anologous to this check during the evaluation of enums:

php-src/Zend/zend_ast.c

Lines 989 to 993 in 4d5b651

	// Preloading will attempt to resolve constants but objects can't be stored in shm
	// Aborting here to store the const AST instead
	if (CG(in_compilation)) {
	return FAILURE;
	}