Fix unused variable that happened during merge

Fix an "unused variable" warning that happened in some configurations
(without EC, found by depend-pkalg.pl) and was not present in any parent PR
but only in the result of merging them: one of the PRs clarified the
distinction between `ret` and `verify_ret` and the other removed one
occurrence of using `ret`, and the conjunction of the two made `ret` unused in
some cases. Resolving by reducing the scope of that variable.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index b53cd35..91b944c 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -6493,7 +6493,6 @@
                                          mbedtls_x509_crt *chain,
                                          void *rs_ctx )
 {
-    int ret = 0;
     int verify_ret;
     const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
         ssl->handshake->ciphersuite_info;
@@ -6543,6 +6542,7 @@
 
 #if defined(MBEDTLS_ECP_C)
     {
+        int ret;
         mbedtls_pk_context *pk;
         ret = mbedtls_x509_crt_pk_acquire( chain, &pk );
         if( ret != 0 )