Make IAR happy by dummy-initializing an unused variable

IAR doesn't like `((void) var);` as a means to indicate an unused
variable if that variable hasn't been initialized before. Make it
happy by initializing the variable before.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index e8c4a09..60bec57 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -3399,7 +3399,7 @@
 
 static int ssl_client_key_exchange_prepare( mbedtls_ssl_context *ssl )
 {
-    int ret;
+    int ret = 0;
     mbedtls_ssl_ciphersuite_handle_t ciphersuite_info =
         mbedtls_ssl_handshake_get_ciphersuite( ssl->handshake );