Fix integer overflow in intset (CVE-2021-29478)

An integer overflow bug in Redis 6.2 could be exploited to corrupt the heap and
potentially result with remote code execution.

The vulnerability involves changing the default set-max-intset-entries
configuration value, creating a large set key that consists of integer values
and using the COPY command to duplicate it.

The integer overflow bug exists in all versions of Redis starting with 2.6,
where it could result with a corrupted RDB or DUMP payload, but not exploited
through COPY (which did not exist before 6.2).

(cherry picked from commit 29900d4e6bccdf3691bedf0ea9a5d84863fa3592)

src/intset.c

@@ -281,7 +281,7 @@ uint32_t intsetLen(const intset *is) {
 
 /* Return intset blob size in bytes. */
 size_t intsetBlobLen(intset *is) {
-    return sizeof(intset)+intrev32ifbe(is->length)*intrev32ifbe(is->encoding);
+    return sizeof(intset)+(size_t)intrev32ifbe(is->length)*intrev32ifbe(is->encoding);
 }
 
 /* Validate the integrity of the data structure.