main: Print WGSL from correct program

PrintWGSL() was using the moved program, triggering an ICE

Change-Id: I8976feac8712cc883d1b1b4bdca35a929bcb846a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53040
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

samples/main.cc

@@ -802,7 +802,7 @@ int main(int argc, const char** argv) {
   }
 
   if (!writer->Generate()) {
-    PrintWGSL(std::cerr, out.program);
+    PrintWGSL(std::cerr, *program);
     std::cerr << "Failed to generate: " << writer->error() << std::endl;
     return 1;
   }