backport ZSTR_MAX_LEN

(cherry picked from commit 37056ad)
(cherry picked from commit ff86894)

Author: remicollet

Zend/zend_string.h

@@ -62,6 +62,9 @@ END_EXTERN_C()
 
 #define _ZSTR_STRUCT_SIZE(len) (_ZSTR_HEADER_SIZE + len + 1)
 
+#define ZSTR_MAX_OVERHEAD (ZEND_MM_ALIGNED_SIZE(_ZSTR_HEADER_SIZE + 1))
+#define ZSTR_MAX_LEN (SIZE_MAX - ZSTR_MAX_OVERHEAD)
+
 #define ZSTR_ALLOCA_ALLOC(str, _len, use_heap) do { \
 	(str) = (zend_string *)do_alloca(ZEND_MM_ALIGNED_SIZE_EX(_ZSTR_STRUCT_SIZE(_len), 8), (use_heap)); \
 	GC_REFCOUNT(str) = 1; \