pnm: Fix spurious error message.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pnm.c

@@ -135,7 +135,7 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
         return -1;
     pnm_get(s, buf1, sizeof(buf1));
     avctx->height = atoi(buf1);
-    if(av_image_check_size(avctx->width, avctx->height, 0, avctx))
+    if(avctx->height <= 0 || av_image_check_size(avctx->width, avctx->height, 0, avctx))
         return -1;
     if (avctx->pix_fmt != PIX_FMT_MONOWHITE) {
         pnm_get(s, buf1, sizeof(buf1));