Add a comment for the added cast with explanation.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

engines/afalg/e_afalg.c

diff --git a/engines/afalg/e_afalg.c b/engines/afalg/e_afalg.c
index 456ffda..1766230 100644
--- a/engines/afalg/e_afalg.c
+++ b/engines/afalg/e_afalg.c
@@ -230,6 +230,10 @@
     memset(cb, '\0', sizeof(*cb));
     cb->aio_fildes = sfd;
     cb->aio_lio_opcode = IOCB_CMD_PREAD;
+    /*
+     * The pointer has to be converted to unsigned value first to avoid
+     * sign extension on cast to 64 bit value
+     */
     cb->aio_buf = (uint64_t)(unsigned long)buf;
     cb->aio_offset = 0;
     cb->aio_data = 0;