Use int ret = MBEDTLS_ERROR_CORRUPTION_DETECTED; idiom

Co-authored-by: Gilles Peskine <gilles.peskine@arm.com>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index fefbd86..ae4882d 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -1147,7 +1147,7 @@
     unsigned char aux_out[MBEDTLS_MD_MAX_SIZE];
     mbedtls_md_context_t aux;
     size_t offset;
-    int ret;
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     mbedtls_md_init( &aux );