mpegvideo_enc: only allocate output packet when we know there will be output

Fixes a memleak.

libavcodec/mpegvideo_enc.c

@@ -1384,6 +1384,17 @@ int ff_MPV_encode_picture(AVCodecContext *avctx, AVPacket *pkt,
     int i, stuffing_count, ret;
     int context_count = s->slice_context_count;
 
+    s->picture_in_gop_number++;
+
+    if (load_input_picture(s, pic_arg) < 0)
+        return -1;
+
+    if (select_input_picture(s) < 0) {
+        return -1;
+    }
+
+    /* output? */
+    if (s->new_picture.f.data[0]) {
         if (!pkt->data &&
             (ret = ff_alloc_packet(pkt, s->mb_width*s->mb_height*MAX_MB_BYTES)) < 0)
             return ret;
@@ -1398,17 +1409,6 @@ int ff_MPV_encode_picture(AVCodecContext *avctx, AVPacket *pkt,
             init_put_bits(&s->thread_context[i]->pb, start, end - start);
         }
 
-    s->picture_in_gop_number++;
-
-    if (load_input_picture(s, pic_arg) < 0)
-        return -1;
-
-    if (select_input_picture(s) < 0) {
-        return -1;
-    }
-
-    /* output? */
-    if (s->new_picture.f.data[0]) {
         s->pict_type = s->new_picture.f.pict_type;
         //emms_c();
         //printf("qs:%f %f %d\n", s->new_picture.quality,