ocl: fix null pointer access crash

modules/core/src/ocl.cpp

@@ -1973,6 +1973,7 @@ struct Context::Impl
     {
         if (prefix.empty())
         {
+            CV_Assert(!devices.empty());
             const Device& d = devices[0];
             prefix = d.vendorName() + "--" + d.name() + "--" + d.driverVersion();
             // sanitize chars
@@ -3222,6 +3223,8 @@ struct Program::Impl
         refcount = 1;
         const Context ctx = Context::getDefault();
         Device device = ctx.device(0);
+        if (ctx.ptr() == NULL || device.ptr() == NULL)
+            return;
         if (device.isAMD())
             buildflags += " -D AMD_DEVICE";
         else if (device.isIntel())
@@ -3232,6 +3235,7 @@ struct Program::Impl
     bool compile(const Context& ctx, String& errmsg)
     {
 #if OPENCV_HAVE_FILESYSTEM_SUPPORT
+        CV_Assert(ctx.getImpl());
         OpenCLBinaryCacheConfigurator& config = OpenCLBinaryCacheConfigurator::getSingletonInstance();
         const std::string base_dir = config.prepareCacheDirectoryForContext(ctx.getImpl()->getPrefixString());
         const std::string fname = base_dir.empty() ? std::string() :