pnm: limit maxval to UINT16_MAX

From 'man ppm': The maximum color value (Maxval), again in ASCII decimal.
                Must be less than 65536.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/pnm.c

@@ -107,7 +107,8 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
             }
         }
         /* check that all tags are present */
-        if (w <= 0 || h <= 0 || maxval <= 0 || depth <= 0 || tuple_type[0] == '\0' || av_image_check_size(w, h, 0, avctx) || s->bytestream >= s->bytestream_end)
+        if (w <= 0 || h <= 0 || maxval <= 0 || maxval > UINT16_MAX || depth <= 0 || tuple_type[0] == '\0' ||
+            av_image_check_size(w, h, 0, avctx) || s->bytestream >= s->bytestream_end)
             return AVERROR_INVALIDDATA;
 
         avctx->width  = w;
@@ -159,7 +160,7 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
     if (avctx->pix_fmt != AV_PIX_FMT_MONOWHITE && avctx->pix_fmt != AV_PIX_FMT_MONOBLACK) {
         pnm_get(s, buf1, sizeof(buf1));
         s->maxval = atoi(buf1);
-        if (s->maxval <= 0) {
+        if (s->maxval <= 0 || s->maxval > UINT16_MAX) {
             av_log(avctx, AV_LOG_ERROR, "Invalid maxval: %d\n", s->maxval);
             s->maxval = 255;
         }