8057830: Crash in Java2D Queue Flusher, OGLSD_SetScratchSurface

Reviewed-by: serb, denis
Contributed-by: NHendrik Schreiber <hs@tagtraum.com>

src/macosx/native/sun/java2d/opengl/CGLGraphicsConfig.m

@@ -72,7 +72,9 @@ OGLGC_DestroyOGLGraphicsConfig(jlong pConfigInfo)
             }
             [pool drain];
             free(ctxinfo);
+            oglc->ctxInfo = NULL;
         }
+        cglinfo->context = NULL;
     }
 
     free(cglinfo);

src/macosx/native/sun/java2d/opengl/CGLSurfaceData.m

@@ -187,6 +187,11 @@ OGLSD_SetScratchSurface(JNIEnv *env, jlong pConfigInfo)
     }
 
     OGLContext *oglc = cglInfo->context;
+    if (oglc == NULL) {
+        J2dRlsTraceLn(J2D_TRACE_ERROR, "OGLSD_SetScratchContext: ogl context is null");
+        return NULL;
+    }
+
     CGLCtxInfo *ctxinfo = (CGLCtxInfo *)oglc->ctxInfo;
 
 JNF_COCOA_ENTER(env);