MDEV-221 : Fix potential memory access past the end of input string in filename_to_tablename()

sql/sql_table.cc

@@ -379,7 +379,7 @@ uint filename_to_tablename(const char *from, char *to, uint to_length
   DBUG_ENTER("filename_to_tablename");
   DBUG_PRINT("enter", ("from '%s'", from));
 
-  if (!memcmp(from, tmp_file_prefix, tmp_file_prefix_length))
+  if (!strncmp(from, tmp_file_prefix, tmp_file_prefix_length))
   {
     /* Temporary table name. */
     res= (strnmov(to, from, to_length) - to);