inspect.c: avoid nulling images in decode loop

BUG=aomedia:2005

Change-Id: I0167bf27ceb5986641a071ba96ddbb7ae4fee837
diff --git a/examples/inspect.c b/examples/inspect.c
index 4887fc4..9d5f0dc 100644
--- a/examples/inspect.c
+++ b/examples/inspect.c
@@ -630,7 +630,9 @@
     die_codec(&codec, "Failed to decode frame.");
   }
   int got_any_frames = 0;
-  while ((img = aom_codec_get_frame(&codec, &iter))) {
+  aom_image_t *frame_img;
+  while ((frame_img = aom_codec_get_frame(&codec, &iter))) {
+    img = frame_img;
     ++frame_count;
     got_any_frames = 1;
   }