Fix posix_kill crashing / killing other programs on null (#68)

When `$pid` in `posix_kill($pid, $signo)` is `null` it kills other programs like Firefox, Terminal, even Gnome Shell.

This patch adds a null check to avoid this.

Discovered via phpactor in Sublime Text: Removing several folders in quick succession causes `posix_kill` to be called with `null`, for whatever reason, which causes a bunch of programs to crash.

See phpactor/phpactor#2516.

Co-authored-by: Niklas Keller <me@kelunik.com>

lib/Internal/Posix/Runner.php

@@ -194,7 +194,7 @@ public function kill(ProcessHandle $handle)
                 return;
             }
             // ignore errors because process not always detached
-            @\posix_kill($pid, 9);
+            self::tryPosixKill($pid, 9);
         });
 
         if ($handle->status < ProcessStatus::ENDED) {
@@ -213,10 +213,17 @@ public function signal(ProcessHandle $handle, int $signo)
                 return;
             }
 
-            @\posix_kill($pid, $signo);
+            self::tryPosixKill($pid, $signo);
         });
     }
 
+    private static function tryPosixKill($pid, int $signo)
+    {
+        if ($pid !== null) {
+            @\posix_kill($pid, $signo);
+        }
+    }
+
     /** @inheritdoc */
     public function destroy(ProcessHandle $handle)
     {