Make sure core module has number 0

Some places, possibly also outside PHP, assume the core extension has
module number 0. After 8a812c3 this wasn't the case anymore as
reported in [1]. Fix it by changing how the next module ID is computed.

[1] php#12246 (comment)

Author: nielsdos

Zend/zend_API.c

@@ -3271,7 +3271,7 @@ ZEND_API void zend_post_deactivate_modules(void) /* {{{ */
 /* return the next free module number */
 ZEND_API int zend_next_free_module(void) /* {{{ */
 {
-	return zend_hash_num_elements(&module_registry) + 1;
+	return zend_hash_num_elements(&module_registry);
 }
 /* }}} */