anim_util: quiet static analysis warning

in ReadAnimatedWebP() frame_index is tied to the reported frame_count.

Change-Id: I69241e5d77a0b8bac1deabd992d0ad2ecf51a4ec

examples/anim_util.c

@@ -232,6 +232,7 @@ static int ReadAnimatedWebP(const char filename[],
       fprintf(stderr, "Error decoding frame #%u\n", frame_index);
       goto End;
     }
+    assert(frame_index < anim_info.frame_count);
     curr_frame = &image->frames[frame_index];
     curr_rgba = curr_frame->rgba;
     curr_frame->duration = timestamp - prev_frame_timestamp;