MDEV-10888: encryption.filekeys_emptyfile fails in buildbot with valgrind

Problem was that length of the filekeys file was not checked and if
length is less than OpenSSL_prefix_len uninitialized memory was
accessed.

mysql-test/suite/encryption/r/filekeys_tooshort.result

+call mtr.add_suppression("Cannot decrypt .*tooshort.enc. Not encrypted");
+call mtr.add_suppression("Plugin 'file_key_management' init function returned error");
+call mtr.add_suppression("Plugin 'file_key_management' registration.*failed");
+FOUND /Cannot decrypt .*tooshort.enc. Not encrypted/ in mysqld.1.err
+create table t1(c1 bigint not null, b char(200))  engine=innodb encrypted=yes encryption_key_id=1;
+ERROR HY000: Can't create table `test`.`t1` (errno: 140 "Wrong create options")
+select plugin_status from information_schema.plugins
+where plugin_name = 'file_key_management';
+plugin_status
+# Test checks if opening an too short filekeys does not crash the server.

mysql-test/suite/encryption/t/filekeys-tooshort.enc

+Salted__
\ No newline at end of file

mysql-test/suite/encryption/t/filekeys_tooshort.opt

+--loose-file-key-management-filekey=secret
+--loose-file-key-management-filename=$MTR_SUITE_DIR/t/filekeys-tooshort.enc
+

mysql-test/suite/encryption/t/filekeys_tooshort.test

+let SEARCH_PATTERN=Cannot decrypt .*tooshort.enc. Not encrypted;
+source filekeys_badtest.inc;
+
+--echo # Test checks if opening an too short filekeys does not crash the server.

plugin/file_key_management/parser.cc

@@ -333,7 +333,7 @@ char* Parser::read_and_decrypt_file(const char *secret)
 
 // Check for file encryption
   uchar *decrypted;
-  if (is_prefix((char*)buffer, OpenSSL_prefix))
+  if (file_size > OpenSSL_prefix_len && is_prefix((char*)buffer, OpenSSL_prefix))
   {
     uchar key[OpenSSL_key_len];
     uchar iv[OpenSSL_iv_len];