avcodec/hw_base_encode: fix use after free on close

The way the linked list of images was freed caused a
use after free, by accessing pic->next after pic was
already freed.

Regression from 48a1a12968

Fix CID1633236

libavcodec/hw_base_encode.c

@@ -804,10 +804,10 @@ int ff_hw_base_encode_init(AVCodecContext *avctx, FFHWBaseEncodeContext *ctx)
 
 int ff_hw_base_encode_close(FFHWBaseEncodeContext *ctx)
 {
-    FFHWBaseEncodePicture *pic;
-
-    for (pic = ctx->pic_start; pic; pic = pic->next)
+    for (FFHWBaseEncodePicture *pic = ctx->pic_start, *next_pic = pic; pic; pic = next_pic) {
+        next_pic = pic->next;
         base_encode_pic_free(pic);
+    }
 
     av_fifo_freep2(&ctx->encode_fifo);