fix: prevent Process::close() double execution

[7-zete-7]

Fixes #116

The Process::close() method contains a block introducing side effects (see

child-process/src/Process.php

Lines 300 to 302 in 1721e2b

	foreach ($this->pipes as $pipe) {
	$pipe->close();
	}

). One of the possible side effects may be a repeated call to the Process::close() method. Due to the deferred protection against double execution of the method (below the block with potential side effects, see

child-process/src/Process.php

Line 310 in 1721e2b

$this->process = null;

), the Process::close() method may be run again. The second execution of the Process::close() method will work correctly, but the first will encounter an error about the absence of a resource in the Process::$process property (this property was set to null during the second execution).

With this change, the protection against double execution of the Process::close() method is moved to the very beginning of the method.