PE: Fix possible crash

In case of an invalid device we may crash when trying
to generate a build directory path.
Prevent this by leaving earlier.

Change-Id: Ieb02abd98467693d38df6dde434e2a6962e4d137
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/projectexplorer/buildconfiguration.cpp

@@ -644,6 +644,8 @@ FilePath BuildConfiguration::buildDirectoryFromTemplate(const FilePath &projectD
     buildDir = buildDir.withNewPath(buildDir.path().replace(" ", "-"));
 
     auto buildDevice = BuildDeviceKitAspect::device(kit);
+    if (!buildDevice)
+        return buildDir;
 
     if (buildDir.isAbsolutePath())
         return buildDevice->rootPath().withNewMappedPath(buildDir);