fix: Check that the result of pcntl_waitpid is the PID of a managed process

The --parallel option spawns processes with pcntl_fork and expects the
result of pcntl_waitpid to be one of its spawned processes. It looks up
the temporary output file of a process in the given $childProcs array
without checking if the returned process ID is actually managed, which
under some circumstances caused by external factors may result in an
undefined array index error.

Fixes #1112

Author: NanoSector

src/Runner.php

@@ -777,7 +777,8 @@ private function processChildProcs($childProcs)
 
         while (count($childProcs) > 0) {
             $pid = pcntl_waitpid(0, $status);
-            if ($pid <= 0) {
+            if ($pid <= 0 || array_key_exists($pid, $childProcs) === false) {
+                // No child or a child with an unmanaged PID was returned.
                 continue;
             }