Fix potential corruption of IV for AES CBC

If passed a zero length, AES CBC could potentially corrupt the passed
in IV by memcpying it over itself. Although this might be ok with
more recent compilers, its not for every compiler we support. Found
by coverity.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/aes.c

@@ -1094,6 +1094,11 @@ int mbedtls_aes_crypt_cbc(mbedtls_aes_context *ctx,
         return MBEDTLS_ERR_AES_BAD_INPUT_DATA;
     }
 
+    /* Nothing to do if length is zero. */
+    if (length == 0) {
+        return 0;
+    }
+
     if (length % 16) {
         return MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH;
     }