avcodec/pnm_parser: Remember the length already scanned for ascii images

Fixes: speed regression with xmap_samsung_gear_2560x1280.pgm
Found-by: Michael Koch
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pnm_parser.c

@@ -19,6 +19,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "libavutil/avassert.h"
 #include "libavutil/imgutils.h"
 
 #include "parser.h" //for ParseContext
@@ -27,6 +28,7 @@
 typedef struct PNMParseContext {
     ParseContext pc;
     int remaining_bytes;
+    int ascii_scan;
 }PNMParseContext;
 
 static int pnm_parse(AVCodecParserContext *s, AVCodecContext *avctx,
@@ -77,17 +79,28 @@ retry:
     } else if (pnmctx.type < 4) {
               uint8_t *bs  = pnmctx.bytestream;
         const uint8_t *end = pnmctx.bytestream_end;
+        uint8_t *sync      = bs;
+
+        if (pc->index) {
+            av_assert0(pnmpc->ascii_scan <= end - bs);
+            bs += pnmpc->ascii_scan;
+        }
 
         while (bs < end) {
-            int c = *bs++;
+            int c;
+            sync = bs;
+            c = *bs++;
             if (c == '#')  {
                 while (c != '\n' && bs < end)
                     c = *bs++;
             } else if (c == 'P') {
                 next = bs - pnmctx.bytestream_start + skip - 1;
+                pnmpc->ascii_scan = 0;
                 break;
             }
         }
+        if (next == END_NOT_FOUND)
+            pnmpc->ascii_scan = sync - pnmctx.bytestream + skip;
     } else {
         next = pnmctx.bytestream - pnmctx.bytestream_start + skip
                + av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);