test/func/malloc: align check as size_t

If the test happens as uintptr_t on CHERI, then we attempt to construct
a capability and use a capability-based test rather than an
integer-based one, and things go south.

src/test/func/malloc/malloc.cc

@@ -21,7 +21,7 @@ void check_result(size_t size, size_t align, void* p, int err, bool null)
     if (our_malloc_usable_size(p) < size)
       abort();
 
-    if (((uintptr_t)p % align) != 0)
+    if (static_cast<size_t>(reinterpret_cast<uintptr_t>(p) % align) != 0)
       abort();
 
     our_free(p);