IPC: QSystemSemaphore/SystemV: move up the check for handle

This avoids recalculating the native key file name every time we call
handle() to ensure the handle exists. The POSIX backend already has the
code like this (whence the comment was copied) and the Win32 backend is
similar.

Change-Id: Idd5e1bb52be047d7b4fffffd175076ec09334ae1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/ipc/qsystemsemaphore_systemv.cpp

@@ -53,6 +53,9 @@ bool QSystemSemaphoreSystemV::runtimeSupportCheck()
  */
 key_t QSystemSemaphoreSystemV::handle(QSystemSemaphorePrivate *self, QSystemSemaphore::AccessMode mode)
 {
+    if (unix_key != -1)
+        return unix_key;  // we already have a semaphore
+
 #if defined(Q_OS_DARWIN)
     if (qt_apple_isSandboxed()) {
         // attempting to use System V semaphores will get us a SIGSYS
@@ -73,9 +76,6 @@ key_t QSystemSemaphoreSystemV::handle(QSystemSemaphorePrivate *self, QSystemSema
         return -1;
     }
 
-    if (-1 != unix_key)
-        return unix_key;
-
     // ftok requires that an actual file exists somewhere
     int built = QtIpcCommon::createUnixKeyFile(nativeKeyFile);
     if (-1 == built) {