libcamera: process: Fix killing innocent processes unexpectedly

When a libcamera::process is being destructed or called kill() without a
previous successful call to start(), it's pid_ may remains -1, which
causes all the killable processes being killed when passed to
`kill(pid_, SIG_KILL)`.

Signed-off-by: You-Sheng Yang <vicamo.yang@canonical.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/process.cpp

@@ -373,7 +373,8 @@ void Process::died(int wstatus)
  */
 void Process::kill()
 {
-	::kill(pid_, SIGKILL);
+	if (pid_ > 0)
+		::kill(pid_, SIGKILL);
 }
 
 } /* namespace libcamera */