Decode file-name to pass as QString to QFile::exists()

QSharedMemorySystemV::updateNativeKeyFile() stores the QBA
nativeKeyFile member using QFile::encodeName(), so feed it back
through QFile::decodeName() rather than relying on QString(QByteArray)
coercion, which might use a different encoding.
Fixes a CodeChecker grumble.

Change-Id: If9fb01ed65b5bba8c1070b4939ee1d87adf94743
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/ipc/qsharedmemory_systemv.cpp

@@ -72,7 +72,7 @@ key_t QSharedMemorySystemV::handle(QSharedMemoryPrivate *self)
     }
 
     // ftok requires that an actual file exists somewhere
-    if (!QFile::exists(nativeKeyFile)) {
+    if (!QFile::exists(QFile::decodeName(nativeKeyFile))) {
         self->setError(QSharedMemory::NotFound,
                        QSharedMemory::tr("%1: UNIX key file doesn't exist")
                        .arg("QSharedMemory::handle:"_L1));