Ensure all EVP calls have their returns checked where appropriate

There are lots of calls to EVP functions from within libssl There were
various places where we should probably check the return value but don't.
This adds these checks.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_lib.c

@@ -3507,8 +3507,11 @@ EVP_MD_CTX *ssl_replace_hash(EVP_MD_CTX **hash, const EVP_MD *md)
 {
     ssl_clear_hash_ctx(hash);
     *hash = EVP_MD_CTX_create();
-    if (md)
-        EVP_DigestInit_ex(*hash, md, NULL);
+    if (*hash == NULL || (md && EVP_DigestInit_ex(*hash, md, NULL) <= 0)) {
+        EVP_MD_CTX_destroy(*hash);
+        *hash = NULL;
+        return NULL;
+    }
     return *hash;
 }