utils: fix deallocated memory in SpawnUnix

The function 'prepare_environment_variable' should allocate data with
new instead of malloc because the free_str_array use delete[].
devs: fix a potential segfault in Coordinator

A null pointer is used into the function to prepare a message in an exception.
vpz: remove useless assignment in CoupledModel

The value stored to the correct variable is never read between the while
and the assignment.
mvle: assign a default value to the number of VPZ
utils: simplify source in SpawnUnix

Since ::execve function does not return in success, we can simplify the
function.