Minor refactor of CVE-2021-32765 fix.

Since `hi_calloc` always passes through one of our wrapper functions,
we can perform this overflow in the wrapper, and get protection
everywhere.

Previous commit: 76a7b10005c70babee357a7d0f2becf28ec7ed1e

Related vuln ID: CVE-2021-32765
[Full Details](https://github.com/redis/hiredis/security/advisories/GHSA-hfm9-39pp-55p2)

1 files changed, 4 insertions, 0 deletions

diff --git a/alloc.c b/alloc.c
index 7fb6b35..0902286 100644
--- a/alloc.c
+++ b/alloc.c
@@ -68,6 +68,10 @@ void *hi_malloc(size_t size) {
 }
 
 void *hi_calloc(size_t nmemb, size_t size) {
+    /* Overflow check as the user can specify any arbitrary allocator */
+    if (SIZE_MAX / size < nmemb)
+        return NULL;
+
     return hiredisAllocFns.callocFn(nmemb, size);
 }