Fix -Wshadow warnings

Backport of ea35666 and 824ba72

see #240
diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index e0f9ae2..7fd254b 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -133,7 +133,7 @@
                            unsigned char *output, size_t len, size_t *olen )
 {
     FILE *file;
-    size_t ret;
+    size_t read_len;
     ((void) data);
 
 #if defined(HAVE_GETRANDOM)
@@ -158,8 +158,8 @@
     if( file == NULL )
         return( POLARSSL_ERR_ENTROPY_SOURCE_FAILED );
 
-    ret = fread( output, 1, len, file );
-    if( ret != len )
+    read_len = fread( output, 1, len, file );
+    if( read_len != len )
     {
         fclose( file );
         return( POLARSSL_ERR_ENTROPY_SOURCE_FAILED );
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index b8fb507..b8a5e6e 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -566,8 +566,6 @@
     }
     else
     {
-        int ret;
-
         /* Initialize HMAC contexts */
         if( ( ret = md_init_ctx( &transform->md_ctx_enc, md_info ) ) != 0 ||
             ( ret = md_init_ctx( &transform->md_ctx_dec, md_info ) ) != 0 )
@@ -1307,7 +1305,7 @@
             /*
              * Generate IV
              */
-            int ret = ssl->f_rng( ssl->p_rng, ssl->transform_out->iv_enc,
+            ret = ssl->f_rng( ssl->p_rng, ssl->transform_out->iv_enc,
                                   ssl->transform_out->ivlen );
             if( ret != 0 )
                 return( ret );