Fix warning on implicit casting in aescrypt.c (#584)

diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index 0270624..c77d77f 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -73,7 +73,7 @@
 {
     int ret = 1;
 
-    int i, n;
+    unsigned int i, n;
     int mode, lastn;
     size_t keylen;
     FILE *fkey, *fin = NULL, *fout = NULL;