Skip special function optimization for redeclared disabled functions

As pointed out on GH-5817.

Author: nikic

Zend/tests/bug79382.phpt

@@ -6,13 +6,11 @@ disable_functions=strlen
 <?php
 
 function strlen(string $x): int {
-    $len = 0;
-    while (isset($x[$len])) $len++;
-    return $len;
+    return 42;
 }
 
 var_dump(strlen("foobar"));
 
 ?>
 --EXPECT--
-int(6)
+int(42)

Zend/zend_compile.c

@@ -3954,6 +3954,12 @@ int zend_try_compile_special_func(znode *result, zend_string *lcname, zend_ast_l
 		return FAILURE;
 	}
 
+	if (fbc->type != ZEND_INTERNAL_FUNCTION) {
+		/* If the function is part of disabled_functions, it may be redeclared as a userland
+		 * function with a different implementation. Don't use the VM builtin in that case. */
+		return FAILURE;
+	}
+
 	if (zend_args_contain_unpack(args)) {
 		return FAILURE;
 	}