pcx: Consume the whole packet if giving up due to missing palette

Previously, we returned 0, meaning successful decoding but 0
bytes consumed, leading to an infinite loop.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 9fb0de86b4)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/pcx.c

@@ -196,6 +196,7 @@ static int pcx_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         }
         if (*buf++ != 12) {
             av_log(avctx, AV_LOG_ERROR, "expected palette after image data\n");
+            ret = buf_size;
             goto end;
         }