Limit unserialization element count more aggressively

This is slightly more aggressive about rejecting obviously incorrect
element counts. Previously the number of elements was allowed to
match the number of characters. Now it is the number of characters
divided by two (this can actually be increased further to at least 4).

This doesn't really matter in the grand scheme of things (as it
just cuts maximum memory usage by half), but should fix
oss-fuzz #29356.

Author: nikic

ext/standard/var_unserializer.re

@@ -29,6 +29,10 @@
 #define VAR_WAKEUP_FLAG 1
 #define VAR_UNSERIALIZE_FLAG 2
 
+/* Each element is encoded using at least 2 characters. */
+#define IS_FAKE_ELEM_COUNT(num_elems, serialized_len) \
+	((num_elems) > (serialized_len) / 2)
+
 typedef struct {
 	zend_long used_slots;
 	void *next;
@@ -1001,7 +1005,7 @@ use_double:
 	*p = YYCURSOR;
     if (!var_hash) return 0;
 
-	if (elements < 0 || elements >= HT_MAX_SIZE || elements > max - YYCURSOR) {
+	if (elements < 0 || elements >= HT_MAX_SIZE || IS_FAKE_ELEM_COUNT(elements, max - YYCURSOR)) {
 		return 0;
 	}
 
@@ -1169,7 +1173,7 @@ object ":" uiv ":" ["]	{
 	}
 
 	elements = parse_iv2(*p + 2, p);
-	if (elements < 0 || elements > max - YYCURSOR) {
+	if (elements < 0 || IS_FAKE_ELEM_COUNT(elements, max - YYCURSOR)) {
 		zend_string_release_ex(class_name, 0);
 		return 0;
 	}