Check for invalid maxval in pgm decoder, fixes issue 2518.

Patch by Daniel Kang, daniel.d.kang at gmail

Originally committed as revision 26292 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/pnm.c

@@ -140,6 +140,10 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
     if (avctx->pix_fmt != PIX_FMT_MONOWHITE) {
         pnm_get(s, buf1, sizeof(buf1));
         s->maxval = atoi(buf1);
+        if (s->maxval <= 0) {
+            av_log(avctx, AV_LOG_ERROR, "Invalid maxval: %d\n", s->maxval);
+            return -1;
+        }
         if (s->maxval >= 256) {
             if (avctx->pix_fmt == PIX_FMT_GRAY8) {
                 avctx->pix_fmt = PIX_FMT_GRAY16BE;