Fix error reporting when terminating process that is not running

Author: clue

src/Process.php

@@ -181,6 +181,10 @@ public function close()
      */
     public function terminate($signal = null)
     {
+        if ($this->process === null) {
+            return false;
+        }
+
         if ($signal !== null) {
             return proc_terminate($this->process, $signal);
         }

tests/AbstractProcessTest.php

@@ -284,6 +284,13 @@ public function testStartAlreadyRunningProcess()
         $process->start($this->createLoop());
     }
 
+    public function testTerminateProcesWithoutStartingReturnsFalse()
+    {
+        $process = new Process('sleep 1');
+
+        $this->assertFalse($process->terminate());
+    }
+
     public function testTerminateWithDefaultTermSignalUsingEventLoop()
     {
         if (defined('PHP_WINDOWS_VERSION_BUILD')) {