GLRenderer: avoid NPE reported by Pavl_G

[stephengold]

This attempts to avoid the NullPointerException reported by Pavl_G at the Forum:
https://hub.jmonkeyengine.org/t/engine-v3-4-0-beta-testing/44507/9

Since this appears to be related to #1408, I'd appreciate a review from @riccardobl

Stack trace from the reported crash:

SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
java.lang.NullPointerException
	at com.jme3.renderer.opengl.GLRenderer.clearVertexAttribs(GLRenderer.java:2855)
	at com.jme3.renderer.opengl.GLRenderer.renderMeshDefault(GLRenderer.java:3188)
	at com.jme3.renderer.opengl.GLRenderer.renderMesh(GLRenderer.java:3219)
	at com.jme3.material.logic.DefaultTechniqueDefLogic.renderMeshFromGeometry(DefaultTechniqueDefLogic.java:72)
	at com.jme3.material.logic.SinglePassAndImageBasedLightingLogic.render(SinglePassAndImageBasedLightingLogic.java:268)
	at com.jme3.material.Technique.render(Technique.java:167)
	at com.jme3.material.Material.render(Material.java:1033)
	at com.jme3.renderer.RenderManager.renderGeometry(RenderManager.java:634)
	at com.jme3.renderer.queue.RenderQueue.renderGeometryList(RenderQueue.java:273)
	at com.jme3.renderer.queue.RenderQueue.renderQueue(RenderQueue.java:318)
	at com.jme3.renderer.RenderManager.renderViewPortQueues(RenderManager.java:918)
	at com.jme3.renderer.RenderManager.flushQueue(RenderManager.java:799)
	at com.jme3.renderer.RenderManager.renderViewPort(RenderManager.java:1128)
	at com.jme3.renderer.RenderManager.render(RenderManager.java:1180)
	at com.jme3.app.SimpleApplication.update(SimpleApplication.java:273)
	at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:537)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:624)
	at com.jme3.system.lwjgl.LwjglWindow.create(LwjglWindow.java:473)
	at com.jme3.app.LegacyApplication.start(LegacyApplication.java:491)

[stephengold]

Unless there's substantial review, I plan to integrate this in 24 hours.

[stephengold]

Thanks for looking, @riccardobl!

[stephengold]

I've now encountered the crash as well, in v3.4.0-beta1:

Apr 25, 2021 3:17:41 PM com.jme3.app.LegacyApplication handleError
SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
java.lang.NullPointerException
	at com.jme3.renderer.opengl.GLRenderer.clearVertexAttribs(GLRenderer.java:2855)
	at com.jme3.renderer.opengl.GLRenderer.renderMeshDefault(GLRenderer.java:3188)
	at com.jme3.renderer.opengl.GLRenderer.renderMesh(GLRenderer.java:3219)
	at com.jme3.material.logic.DefaultTechniqueDefLogic.renderMeshFromGeometry(DefaultTechniqueDefLogic.java:72)
	at com.jme3.material.logic.DefaultTechniqueDefLogic.render(DefaultTechniqueDefLogic.java:97)
	at com.jme3.material.Technique.render(Technique.java:167)
	at com.jme3.material.Material.render(Material.java:1033)
	at com.jme3.renderer.RenderManager.renderGeometry(RenderManager.java:634)
	at com.jme3.renderer.queue.RenderQueue.renderGeometryList(RenderQueue.java:273)
	at com.jme3.renderer.queue.RenderQueue.renderQueue(RenderQueue.java:318)
	at com.jme3.renderer.RenderManager.renderViewPortQueues(RenderManager.java:918)
	at com.jme3.renderer.RenderManager.flushQueue(RenderManager.java:799)
	at com.jme3.renderer.RenderManager.renderViewPort(RenderManager.java:1128)
	at com.jme3.renderer.RenderManager.render(RenderManager.java:1180)
	at com.jme3.app.SimpleApplication.update(SimpleApplication.java:273)
	at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:537)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:624)
	at com.jme3.system.lwjgl.LwjglWindow.create(LwjglWindow.java:473)
	at com.jme3.app.LegacyApplication.start(LegacyApplication.java:491)
	at com.jme3.app.LegacyApplication.start(LegacyApplication.java:443)
	at com.jme3.app.SimpleApplication.start(SimpleApplication.java:126)
	at com.jayfella.jme.vehicle.simpledemo.HelloMav.main(HelloMav.java:93)

Glad to say the fix has been committed to both the "master" and "v3.4" branches.