Fix incorrect closing bracket in strlen call

The bracketing was causing heap overflow when trying to run the system
tests.

maxscale-system-test/nodes.cpp

@@ -328,7 +328,7 @@ int Nodes::read_basic_env()
             }
             else
             {
-                access_homedir[i] = (char *) malloc(strlen(access_user[i] + 9));
+                access_homedir[i] = (char *) malloc(strlen(access_user[i]) + 9);
                 sprintf(access_homedir[i], "/home/%s/", access_user[i]);
             }